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Deep geothermal drilling is a necessary technological stage to produce renewable energy by "enhanced geothermal systems" everywhere. However, the high cost and complexity of deep drilling through hard rock formations is a major barrier to its commercialization. One approach to reach affordability and reduce the cost of deep drilling is to use millimeter-wave (mmw) gyrotron radiation to melt or vaporize rocks. In this paper, the results of an experimental study of mmw radiation absorption by hard rocks are presented. Electromagnetic attenuation and reflection were measured in W-band at frequencies from 75 to 110 GHz in eight rock samples from six different geological formations originated in Israel. The results show that the mmw radiation absorption in hard rocks ranges from 90 to 99% per centimeter. The absorption varies slightly with rock type and is relatively independent of frequency but has reached higher values at the upper W band for all samples. The study suggests that high-power mmw radiation is a promising technology for deep geothermal drilling. The results of this experiment, attenuation, and reflection of W-band radiation in hard rocks provide valuable information for the design and optimization of mmw-based drilling systems, and will support the design and optimization of gyrotron based hard rock drilling equipment for geothermal applications. [ABSTRACT FROM AUTHOR]

Lost circulation is a critical issue in drilling. This work reports the investigations on shape memory epoxy polymers (SMEPs) with high switching temperatures, and good thermal, mechanical and shape memory properties for plugging in deep drilling. The SMEPs are produced by curing diglycidyl ether of bisphenol A (DGEBA) and castor oil glycidyl ether (COGE) with a flexible poly(oxypropylene) diamine (D230) and a rigid 4, 4′ ‐diaminodiphenyl methane (DDM). Increasing D230 contents or using COGE, glass transition temperature (Tg, 93–163 °C, DMA analyses) and crosslink density of SMEPs are reduced, shape recovery rate is increased, while tensile strength, elongation at break and impact strength are improved. Further, shape memory epoxy syntactic foams (SMEFs) with high‐temperature adhesion are developed by introducing polymer microspheres into SMEPs. The SMEFs have compression ratio of 52–58%, and expansion ratio of 100% by adding 2% wt. microspheres. They retain the Tg and thermal stability of SMEPs, and show a delayed, slower recovery than SMEPs. Long fracture plugging experiments exhibit that a pressure‐bearing capability of 8.99–9.81 MPa is achieved with SMEFs in sealing fracture slots at a high temperature. [ABSTRACT FROM AUTHOR]

Three high-temperature resistant polymeric additives for water-based drilling fluids are designed and developed: weakly cross-linked zwitterionic polymer fluid loss reducer (WCZ), flexible polymer microsphere nano-plugging agent (FPM) and comb-structure polymeric lubricant (CSP). A high-temperature resistant and high-density polymeric saturated brine-based drilling fluid was developed for deep drilling. The WCZ has a good anti-polyelectrolyte effect and exhibits the API fluid loss less than 8 mL after aging in saturated salt environment at 200 °C. The FPM can reduce the fluid loss by improving the quality of the mud cake and has a good plugging effect on nano-scale pores/fractures. The CSP, with a weight average molecular weight of 4804, has multiple polar adsorption sites and exhibits excellent lubricating performance under high temperature and high salt conditions. The developed drilling fluid system with a density of 2.0 g/cm3 has good rheological properties. It shows a fluid loss less than 15 mL at 200 °C and high pressure, a sedimentation factor (SF) smaller than 0.52 after standing at high temperature for 5 d, and a rolling recovery of hydratable drill cuttings similar to oil-based drilling fluid. Besides, it has good plugging and lubricating performance.

Abstract Lost circulation is a critical issue in drilling. This work reports the investigations on shape memory epoxy polymers (SMEPs) with high switching temperatures, and good thermal, mechanical and shape memory properties for plugging in deep drilling. The SMEPs are produced by curing diglycidyl ether of bisphenol A (DGEBA) and castor oil glycidyl ether (COGE) with a flexible poly(oxypropylene) diamine (D230) and a rigid 4, 4′ ‐diaminodiphenyl methane (DDM). Increasing D230 contents or using COGE, glass transition temperature (Tg, 93–163 °C, DMA analyses) and crosslink density of SMEPs are reduced, shape recovery rate is increased, while tensile strength, elongation at break and impact strength are improved. Further, shape memory epoxy syntactic foams (SMEFs) with high‐temperature adhesion are developed by introducing polymer microspheres into SMEPs. The SMEFs have compression ratio of 52–58%, and expansion ratio of 100% by adding 2% wt. microspheres. They retain the Tg and thermal stability of SMEPs, and show a delayed, slower recovery than SMEPs. Long fracture plugging experiments exhibit that a pressure‐bearing capability of 8.99–9.81 MPa is achieved with SMEFs in sealing fracture slots at a high temperature.

Oil-based drilling fluids for deep well drilling should have stability and high temperature resistance, and water-in-oil inverse emulsions are a good choice as oil-based drilling fluids. In deep well drilling, high temperature will cause polymer surfactants to degrade and phase separation occurs. The octadecyltrimethoxysilane modified silica nanoparticles are used to form a stable water-in-oil inverse emulsion in a drilling fluid model, so that the emulsion fluid forms 60 µm water droplets. In addition, rheology shows that the use of hydrophobic silica nanoparticles can obtain stable water-in-oil inverse emulsions, the properties of which can be modified by adjusting the properties and content of the nanoparticles. The aging test at 120 °C shows that it has good stability at high temperatures and is suitable for deep well drilling operations. [ABSTRACT FROM AUTHOR]

The aim of the article is to find a correlation between a change in the properties of the cooling agent and a change in the geometry of the chipformer, as both are reflected in the service life of the tool after deep drilling. The reason for carrying out the research is the requirement of practice to obtain the economic efficiency of the production of such a demanding process as deep drilling. When applying the latest designs of gun drills, it is very important to correctly set the technological parameters to maintain the stability of the cutting process. One of the most important parameters is the correct removal of heat from the cutting site, and this will be ensured by the stability of pressure, temperature, and percentage of emulsion in the cooling medium, as well as the adjustment of the geometry of the chip former. On this basis, a large number of tests were carried out, consisting of testing the number of drilling cycles carried out by new, unfluted gun drills at constant feed rates and spindle rotation frequencies. After testing, it is possible to modify and supplement the existing methodology of deep drilling technology in terms of managing the cooling emulsion and chip-forming geometry. The tests were aimed at increasing the service life and the number of possible re-grindings of the gun drills depending on changes in the percentage of the emulsion concentration, the pressure and temperature of the cooling agent, and the associated chipformer geometry. [ABSTRACT FROM AUTHOR]

Copyright of Geology & Exploration is the property of Geology & Exploration Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Titanium and its alloy are materials that attract great interest due to mechanical properties, such as density, mechanical strength, and others. Titanium has been demonstrated as an excellent material to be used as a biomaterial. The field of biomaterials, mainly in prostheses, has continuously increased due to the high demand for people with injuries due to accidents or ageing. However, manufacturing these materials requires great care due to the difficulties on the surface to produce good osseointegration. Among the several machining processes, drilling is the most popular process in the manufacturing sector. Nevertheless, the drilling has serious challenges, mainly for the titanium and its alloys. The present work developed experimental and numerical tests of deep drilling in the Ti-6Al-4 V ELI titanium alloy, analysing the cutting forces, geometric tolerance, and temperature. The finite element method using a 3D numeric model was applied to predict the outcomes, and experimental tests were carried out with solid carbide helical drills to confirm the results. The results indicated that using high feed could be a great alternative in drilling Ti-6Al-4 V ELI. This technique can improve productivity and attend to the demands in the biomaterials area within certain tolerance classes. [ABSTRACT FROM AUTHOR]

In this study, the spatial variability of nitrogen (N) balances and potential nitrate leaching were determined in heterogeneous arable fields in southern Germany using digital methods (tractor-mounted multispectral sensor, satellite data, vegetation indices and models) and measurements of nitrate stocks in deeper zones (1−2.5 m soil depth) with deep drilling. The aim of this study was to achieve precise localisation of N losses at high spatial resolution (sub field level; 10 × 10 m grid). The spatial variability of plant parameters within uniformly fertilised fields (crop yield, N uptake) was determined for the calculation of the N balances. The spatial variability of soil properties (e.g. soil organic carbon content, soil total nitrogen content) were determined to identify the causes of high or low N surpluses and nitrate leaching in sub-fields. N surplus determined using different digital methods and measured nitrate stocks showed similar spatial patterns. Site-specific N balancing identified zones with high N loss potential (N surplus up to 86 kg ha−1). Deep drilling showed zones with high nitrate loss (nitrate N up to 94 kg ha−1). N surplus and nitrate loss correlated with r = 0.49. This relationship could be impacted by many other soil and management factors. Soil properties showed considerable spatial variation within the fields. Soil organic carbon (SOC) and soil total nitrogen (TN) content were closely correlated in all fields (up to r = 0.96) and were most closely positively correlated with crop yield and N uptake (up to r = 0.74) and negatively correlated with N surplus (up to r = − 0.73). The sensor data and satellite data (crop yield, N uptake, N surplus) showed similar distribution patterns. Based on these results, digital technologies are suitable for the calculation of site-specific N surplus and estimation of nitrate leaching risk. Satellite or sensor based site-specific and yield-oriented fertilization is one approach to reduce N surplus on sub fields with low yield potential and high nitrate leaching risk. [ABSTRACT FROM AUTHOR]

The aim of the Iceland Deep Drilling Project is to drill into supercritical geothermal systems and examine their economic potential. The exploratory well IDDP-2 was drilled in the Reykjanes geothermal field in SW Iceland, on the landward extension of the Mid-Atlantic Ridge. The Reykjanes geothermal field produces from a

The aim of the Iceland Deep Drilling Project is to drill into supercritical geothermal systems and examine their economic potential. The exploratory well IDDP-2 was drilled in the Reykjanes geothermal field in SW Iceland, on the landward extension of the Mid-Atlantic Ridge. The Reykjanes geothermal field produces from a

Scientific and technological progress over the last century has led to an enormous increase in the consumption of minerals, including energy resources. Most of the exploited oil and gas fields are already considerably depleted, so it is necessary to search for new hydrocarbon resources, particularly at great depths. Deep drilling plays a special role in solving this problem. The article considers the world and Russian experience of ultra-deep wells drilling. The me-thods and technologies used in the construction of wells, as well as complications and accidents occurring during their drilling were analyzed. The analysis revealed that the existing limitations for drilling parameters of deep and ultra-deep wells are caused by the technical characteristics of surface and bottomhole drilling equipment, which do not meet the extreme drilling conditions. The directions for development of deep and ultra-deep well drilling machinery and technologies are suggested. The notion of extreme rock and geological drilling conditions is introduced, which describes drilling in conditions of hydrostatic pressure of flushing fluid column and high bottomhole temperature both at stable and unstable wellbore conditions, coming close to the upper limit of operating technical characteristics of bottomhole assembly, the drill string and flushing fluid.

In Switzerland, the Opalinus Clay unit was chosen as host rock for a repository for nuclear waste and has recently been investigated in a deep drilling campaign at possible repository construction sites. X-ray images of drill cores were compiled into virtual rock columns and were statistically analyzed with respect to layered compositional variations. This provides insight into scale-dependent homogenization and improves sampling strategy. To predict the repository behavior, using continuum-based models of Opalinus Clay, requires the knowledge of effective properties related to a minimum volume at which Opalinus Clay behaves homogeneously. It turned out that with respect to rock composition, such a volume does not exist in the sense that a single sample of manageable size provides a reliable mean composition. This is because the variation of the cm to dm thick layers, which differ in composition, does not sufficiently homogenize even at the 10-m scale. Thus, effective properties must be obtained by averaging several handleable samples. Regarding the composition of Opalinus Clay at a particular location, about 30 samples, distributed over the whole thickness, with a length of about 30 cm should be measured so that the relative error of the mean value is not higher than 5%–10%. For the statistical analyses computed tomography (CT) values of X-ray data were calibrated with respect to rock composition based on laboratory measurements. The CT values are largely controlled by the respective volume fraction of calcite, quartz, and porous clay matrix. These three components form >80 vol% of the sedimentary rocks studied (also above and below Opalinus Clay). The relationship between CT value and component contents depends on the rock type. The use of data from different rock types to calibrate CT values with respect to composition can lead to erroneous results.

The aim of the article is to find a correlation between a change in the properties of the cooling agent and a change in the geometry of the chipformer, as both are reflected in the service life of the tool after deep drilling. The reason for carrying out the research is the requirement of practice to obtain the economic efficiency of the production of such a demanding process as deep drilling. When applying the latest designs of gun drills, it is very important to correctly set the technological parameters to maintain the stability of the cutting process. One of the most important parameters is the correct removal of heat from the cutting site, and this will be ensured by the stability of pressure, temperature, and percentage of emulsion in the cooling medium, as well as the adjustment of the geometry of the chip former. On this basis, a large number of tests were carried out, consisting of testing the number of drilling cycles carried out by new, unfluted gun drills at constant feed rates and spindle rotation frequencies. After testing, it is possible to modify and supplement the existing methodology of deep drilling technology in terms of managing the cooling emulsion and chip-forming geometry. The tests were aimed at increasing the service life and the number of possible re-grindings of the gun drills depending on changes in the percentage of the emulsion concentration, the pressure and temperature of the cooling agent, and the associated chipformer geometry.

This paper represents a new lubricant method which is able to one-stroke drill deep holes with a length-to-diameter of 8, on the AISI SUS 304 stainless steel. By adding graphene nanosheet into typical soluble emulsion and then mixing with water, a nano fluid can be made. The results revealed that using nanofluid can provide a reduction of 4.4-fold of the drilling torque, and thus expand the tool life as many as 20 times, compared with using typical emulsion lubricant. The proper set of cutting parameters was found by using Taguchi L9 experiments as 550 rpm spindle speed and 0.05 mm/rev. The results can be expanded to apply in other deep drilling of hard-to-cut material, using inexpensive devices and avoiding peck-drilling. The proposed lubricant can also be promissing for other machining operations. [ABSTRACT FROM AUTHOR]

The machining of a deep hole by a gun drill is considered, both theoretically and experimentally. The cutting zone is divided notionally into 20 annular layers, for each of which the cutting parameters are calculated by successive approximation. A formula permits estimation of the cutter's wear facet, taking account of the energy consumed, the tool parameters, and the machining time. [ABSTRACT FROM AUTHOR]

Copyright of Oil Drilling & Production Technology / Shiyou Zuancai Gongyi is the property of Shiyou Zuancai Gongyi Bianjibu and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper represents a new lubricant method which is able to one-stroke drill deep holes with a length-to-diameter of 8, on the AISI SUS 304 stainless steel. By adding graphene nanosheet into typical soluble emulsion and then mixing with water, a nano fluid can be made. The results revealed that using nanofluid can provide a reduction of 4.4-fold of the drilling torque, and thus expand the tool life as many as 20 times, compared with using typical emulsion lubricant. The proper set of cutting parameters was found by using Taguchi L9 experiments as 550 rpm spindle speed and 0.05 mm/rev. The results can be expanded to apply in other deep drilling of hard-to-cut material, using inexpensive devices and avoiding peck-drilling. The proposed lubricant can also be promissing for other machining operations.

• A novel torque analysis method for drilling deep lunar soil by DEM is presented. • Drilling models for the bit and stem are built separately using the idea of equivalence. • A torque synthesis method is proposed to get the basic torque at different depths. • The prediction curve of 1.4 m drilling depth was compared with the experiment curve. • This method can save the simulation time greatly for engineers to design the torque. In order to solve the torque design problem of deep lunar soil sampling using drilling, a novel torque analysis method was presented based on discrete element model (DEM). This method includes three stages: drilling simulation of the bit and stem segment, resultant torque calculation, and predicted curve fitting. First, special drilling models were designed for a bit and stem separately. A high-density equivalent particle group, boundary vibration control, pre-drilling simulation and constant pressure surface control were designed for the bit and stem drilling modelling at different depths to ensure the rationality of the model. An example of the torque synthesis process was given, and the simulation time was analyzed. Finally, the simulation predicted torque curve was plotted and compared with the experimental curve. The experimental and simulation curves show that as the drilling depth increases, the torque increases approximately linearly first and then flattens out gradually after a depth of 1 m. The consistency between the two results indicated that the proposed method was validated. Using this method, engineers can take short time to analyze the torque and design basic parameters of the drill mechanism. The problem of high experimental cost and long simulation time in torque design is solved. [ABSTRACT FROM AUTHOR]

Deep drilling of structural timber using ultra short pulse laser ablation is investigated directed towards using this for a novel approach for reinforcement and connection of structural timber. To gain a basic understanding of the interaction processes, the optical properties as well as the different ablation behavior of early and late wood are investigated and discussed. Based on these findings a hypothesis is formulated that directly links the local cell density with the ablation behavior of wood. Furthermore, deep drilling processes are successfully applied to structural timber despite inhomogeneities of the material. We show that the damaging effects of the laser drilling on the mechanical properties of the realized screw connections is negligible. [ABSTRACT FROM AUTHOR]

Features of the mathematical modeling of drilling tools modernized for drilling deep holes are studied. A mathematical model of drilling, questions of stability, and existence of the drilling process are considered.

This article presents a new practical approach to gain a better insight into the machinability enhancements of ultrasonic-assisted drilling (UAD) of aluminum alloys where the depth ratio is larger than 10. UAD can provide a depth as up to two times compared to that of conventional drilling (CD). The material removal rate of UAD appeared higher than that of CD by 1.5 times. The total drilling torque was considered to consist of three components: the cutting torque, the rubbing torque and the stick-slip torque. Both rubbing torque and stick-slip torque magnitude in CD were higher several times compared to the cutting torque. In contrast, such components in UAD took a small proportion of the cutting torque. Growing rate of the rubbing torque at deeper holes in UAD was slower than that in CD about 33 times. The effect of ultrasonic assistance on the reduction of chip resistance was found to be more effective at high depth ratio. The new approach gives better insight to understand the effects of ultrasonic assistance in deep drilling and would further develop to apply in similar studies. [ABSTRACT FROM AUTHOR]

In order to enhance the understanding of the planet's geological evolution and determine whether there exists liquid water, solid ice or any other life evidence, plenty of extraterrestrial explorations have been attempted over the past decades. For future planetary drilling missions, especially unmanned drilling tasks, the autonomous drilling activity should be first verified. In order to replicate the geological formations encountered in the lunar drilling process, large amounts of lunar soil simulant are required. This simulant must be prepared at controlled densities in order to mimic the true mechanical behavior of real lunar soils. Herein, a tamping based dense soil simulant preparation method is proposed. Taking hammer's rebounding, soil's plastic-elastic deformation, and simulant's effective dense height into consideration, a novel theoretical tamping model has been established to predict the preparation parameters. Verification tests showed that the theoretical model correctly predicts the experimentally observed trend in the tamping force with the number of cycles and the optimized relative density of simulant over 180 mm depth from the top is close to that of the real deep lunar soil. • A tamping based dense soil simulant preparation method is proposed. • A theoretical model is established to predict preparation parameters. • Experiments verified the theoretical model by similar tamping force trend. • Under optimized cycles, over 180 mm depth's simulant has been acquired. [ABSTRACT FROM AUTHOR]

Featured Application: Deep-hole drilling processes. This paper presents a new approach to developing the torque model in deep hole drilling, both for conventional and ultrasonic assisted drilling processes. The model was proposed as a sum of three components: the cutting, the chip evacuation and the stick-slip torques. Parameters of the new model were carried out by applying the regression analysis technique, with the correlation values higher than 0.999. The data were collected from 36 experimental dry drilling tests, both in conventional and ultrasonic assisted cutting conditions, with the depth-to-diameter of the drilled holes of 7.5. The major advantage of the new model compared to previous models is that the new model of chip-evacuation torque has only one coefficient, thus making it easier to evaluate and compare different deep-drilling processes. The effectiveness of ultrasonic assistance in deep hole drilling was also highlighted using the proposed model. The new model is promising to predict critical depth and torque in deep hole drilling. [ABSTRACT FROM AUTHOR]

Copyright of Oil Drilling & Production Technology / Shiyou Zuancai Gongyi is the property of Shiyou Zuancai Gongyi Bianjibu and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Oil Drilling & Production Technology / Shiyou Zuancai Gongyi is the property of Shiyou Zuancai Gongyi Bianjibu and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper presents an experimental study of the positive effects of vibration-assisted deep drilling of aluminum alloy Al-6061. The four most important evaluation criteria in drilling—machinability, workpiece temperature, torque, and material removal rate—were chosen to be investigated. Holes with a depth-to-diameter ratio of 13 were drilled by high speed steel (HSS) twist drill bits of 3 mm diameter, using both methods of conventional drilling (CD) and ultrasonic-assisted drilling (UAD). Three levels of axial force of 6 kgf, 9 kgf, and 12 kgf were kept constant for each pair of comparison experiments. It was found that workpiece temperature and torque not only changed from one drill to the other, but were also dependent on the hole depth being drilled. Comparisons were made in-pair between CD and UAD under the same axial force and at the same order of hole numbers. The result shows that the material removal rate with UAD was up to 3.5 times higher than that with CD and the average workpiece temperature and torque in UAD were reduced by 3.5 and 6 times, respectively. Moreover, tool life in UAD was observed to increase from 2.5 to 5 times, in terms of number of holes drilled, compared to that in CD. [ABSTRACT FROM AUTHOR]

>98% of Antarctica is covered by ice sheet with an average thickness of 2126 m. The maximum thickness is 4897 m. Several scientific deep-drilling expeditions are being conducted in Antarctica, including studies on the evolution of Earth's paleoclimate, searches for microbiological life inhabiting deep ice, estimates of Antarctic geothermal heat flux, and observations of subglacial environments. Such operations are complicated by extremely harsh environments; thus, special purpose-built drilling equipment and technology have been designed, and conventional drilling rigs have been heavily modified. Electrothermal and electromechanical drills are suspended on cables if continuous core sampling is required, and hot-water drilling systems for non-core boring are routinely used to drill deep boreholes in the Antarctic ice. The depth of the deepest hole was 3769.3 m at the Vostok Station. Projects with cable-suspended drills require a long time to complete. Hot-water drilling systems are much faster but are very heavy and bulky and consume a large amount of power. To develop lighter and energy-efficient drilling technologies, ice-drilling communities are continuously investigating innovative ice-drilling systems. This paper summarizes the current state of knowledge with respect to the design and performance of various tools and drills for deep drilling in Antarctic ice and presents prospects for improving existing drilling methods and developing new ones. [ABSTRACT FROM AUTHOR]

An alternative drilling technology has been developed, in which ultra-high pressure jetting with in excess of 2500 bar jet pressure is combined with mechanical drilling techniques. In order to investigate the cutting performance of the fluid jets in hard rock formations, experiments under various ambient conditions were performed. Core samples from several drilling sites were tested under atmospheric conditions and enabled a solid basis for the comparison of the jettability of different hard rock formations. To simulate deep downhole conditions as realistically as possible, a pressure vessel capable of up to 450 bar internal pressure was designed and built. The experiments, performed under different ambient pressure regimes, show a completely different cutting performance than under atmospheric conditions. The main influencing parameters were determined and adapted to enable a sufficient performance for all tested conditions. Furthermore the usage of drilling fluids in place of water was investigated. The study shows that high pressure jetting is feasible in the challenging simulated downhole environment, including high ambient pressure, several jets with different pressures acting simultaneously, drilling mud as jetting resp. surrounding fluid and high traverse velocities. [ABSTRACT FROM AUTHOR]

A technology of producing hollow railroad axles by screw piercing and radial forging is developed. The design of hollow axles and the production technology are described. The sequence of piercing and forging operations is presented. Trial rough axles were produced. Their mechanical properties are tested, and the test results are compared to the requirements to solid axles. [ABSTRACT FROM AUTHOR]