Thread Pitch Research Articles

Geometrical non-linear thermal stress analysis of threaded-adhesive joints applied in non-magnetic dewar

The geometrical non-linear thermal stress analysis of the threaded-adhesive joints for non-magnetic Dewar was carried out. The stress analysis shows that the serious stress concentration occurs around the adhesive fillet at the adhesive free end of the lower threaded–adhesive joint of the inner shell. The glass fiber reinforced plastics (GFRP) around the roots of the first and last threads of the internal thread of the joint are also critical regions. In addition, the effects of the thread length and the thread pitch on the von Mises stress distribution were discussed. Decrease of the thread length and increase of the thread pitch will decrease the maximum von Mises stress at the critical locations of the adhesive and GFRP.

Use of artificial neural networks for determining optimum thread forming speeds for thread forming fasteners

PurposeDevelops a neural network based system for optimum assembly speeds using thread‐forming fasteners.Design/methodology/approachUses a three layer neural network to optimise thread forming speeds based on thread diameter and pitch and the total number of thread coils.FindingsThe method demonstrates savings in energy and reduction in torque values of 20‐30 per cent.Research limitations/implicationsProvides a method that works even when less experimental data are available.Practical implicationsThe method should provide a higher quality and reliability and allow thread‐forming fasteners to be used in new application areas.Originality/valueProvides an efficient and less labour intensive method for insertion speed optimisation.

Biomechanical effect of abutment on stability of orthodontic mini‐implant

The biomechanical influences of primary factors on titanium mini-implant, which is used as an anchorage for orthodontic tooth movement, were quantified using the three-dimensional finite element method. Six types of finite element models were designed to show various thread pitches from 0.5 to 1.5 mm. Three models were designed with abutment and three other models without abutment. A traction force of 2 N was applied to the head of the mini-implant or abutment to be at 45 degrees to the bone surface. No remarkable differences were observed in the stress distribution patterns regardless of thread pitch variance. However, the stress distribution was remarkably different between models with abutment and without abutment. The maximum stress of the model with abutment and thread pitch 0.5 mm was the least as compared with the other models. Areas of high-level stress were obviously smaller than in the models without abutment. The plots of the displacement distributions of the models with abutment also presented significant pattern differences as compared with the models without abutment. The high-level area was localized to the head of the implant and the abutment in models with abutment. Therefore, the existence of the abutment is significantly useful in decreasing the stress concentration on the bone, while the effect of thread pitch was uncertain.

An Experimental Study of Bearing and Thread Friction in Fasteners

An experimental procedure is proposed for studying the underhead and thread friction in fasteners. The effective bearing friction radius, the underhead friction coefficient, and the thread friction coefficient are experimentally determined for fasteners with standard hexagonal heads and for flanged head fasteners. Hence, greater accuracy has been achieved in determining the value of the torque components that are consumed in overcoming friction in threaded fasteners. This would lead to a more reliable torque-tension correlation and would enhance the safety and quality of bolted assemblies. A design of experiment procedure is presented in order to investigate the effect of fastener material class, the thread pitch, and the fastener size on thread friction coefficient. For the underhead bearing friction, an experimental model is presented in order to determine the effect of the radii ratio of the contact area on the bearing friction radius.

Experimental evaluation of the effect of thread pitch on fatigue life of bolts

The effect of thread pitch (coarse and fine threads) on fatigue life of ISO bolts within the range of 10≤ D≤24 mm ( D=outside diameter) and American Unified bolts within the range of 7/16″≤ D≤1″ was studied. The experimental results showed that ISO standard coarse threaded bolts have a higher fatigue life than the fine threaded bolts. For unified bolts, however, when the comparison is made on the basis of core diameter, coarse threaded bolts are superior to the fine ones, whereas on a nominal based comparison, both coarse and fine threaded bolts exhibit the same loading capacity. The results also indicated that for the same core stress, the fatigue life of both ISO and unified bolts, except for 1″ unified bolts, decreases as the nominal diameter increases within the range tested. For 1″ unified bolts, the fatigue life increases with the increase of nominal diameter.

Application of digital image analysis method in metric screw thread metrology

This paper describes the use of computer-aided measurement for external metric screw threads. Thread parameters, including thread pitch, thread angle, pitch diameter and major diameter, were measured with CCD cameras and image analysis software. New technologies such as digital image processing were used to increase the efficiency of measurements. In this study, by reconstructing the toolmaker's microscope, the computer-aided semi-automated measuring system was developed, which could evaluate the accuracy of screw thread profile. It is concluded that the measurement accuracy is comparable to that of traditional toolmaker's microscope method.

Prediction of Thread Quality by Detection and Estimation of Tapping Faults

A method is proposed for predicting the internal thread height as a function of tapping process faults and relating it to the standardized thread quality tolerances. Based on the mechanistic tapping model [6], a method of estimating the magnitudes of the faults has been proposed. The effects of tap-hole axes misalignment and tap runout on thread height have been revealed by the mechanistic tapping process model. Based on this model and data obtained from the process estimated process fault values have been used to predict thread height and then predict thread pitch diameter. Using standard tolerances for pitch diameter, an assessment of thread quality can then be made. Validation experiments have been run on several materials for different combinations of process faults. Results of thread measurements showed excellent correspondence with the model predictions.

Single-mode fiber-based reflex sensor for internal surface in-line measurement of small products

Abstract This paper describes experiments concerning dimension and surface shape measurement with a single-mode fiber-optic sensor and improvement of the measurement principle. The sensor probe was well designed with a small structure and a compensation network. The novel optical fiber probe arrangement as well as the possibilities for complex measurement problems is explained by measuring an internal screw thread as an example. The size of the fiber-based sensor probe is about 5 mm, which is very suitable for the internal surface measurement of small precision products. The experimental results show that the measurement uncertainty of the internal screw thread minor diameter can reach ±10 μm, and the stability of the measurement system is better than 0.1%. Compared the measurement results obtained by the developed sensor system with those by a Universal Tool Maker’s Microscope, the measurement results of an external screw thread are satisfactory. The maximum deviation of the thread pitch measurement is about 1 μm, and that of the major diameter measurement is better than 3 μm.

Analysis of Feed Errors in Tapping by Contact Stress Model

One of the most common tapping errors, mismatch of tap feed rate and the thread pitch, has a significant effect on the tapping thrust signal and the resultant thread profile. The underfeed of the tap leads to an upward pull on the workpiece and overfeed causes a downward push. Tapping experiments are conducted with intentionally magnified feed errors to study the phenomenon and aid in the model development. Using contact stress theory for elastic materials, a model for tap-workpiece interaction is developed and integrated with the mechanistic force model developed earlier to predict the tapping forces. In most cases there was a good agreement between the simulations of combined model (contact stress model coupled with mechanistic model) and the experimental results. Tap tooth design with relief on the flank has been shown to significantly reduce the tapping thrust sensitivity to the feed error, thereby affecting the tool life and improving the thread quality.

Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out

Background context: Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better “fit and fill” of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion.Purpose: To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion. Study design/setting: This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format. Methods: Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student's t test. Results: Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P<.10) and a 50% increase in initial stiffness (P<.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion. Conclusions: Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter.

Mechanistic Model for Tapping Process With Emphasis on Process Faults and Hole Geometry

A mechanistic approach for modeling the tapping process is presented. A methodology for computing chip load is developed for an arbitrary tap geometry. The mechanics of cutting for tapping is analyzed, considering it as an oblique cutting phenomenon. The effects of tap geometry (tap diameter, thread pitch, number of flutes, flute helix angle, tooth rake angle, and thread type), workpiece geometry (hole diameter and hole depth), process parameters (spindle speed and tap penetration depth), and process faults (tap runout, axis misalignment, and drilled hole geometry) are incorporated in the model. The model is calibrated using drilling experiments and is validated by comparing experimental tapping results for aluminum 319, aluminum 356, and gray cast iron. In most cases, the tapping forces were predicted within 10 percent of the experimental values.

The Ankylos Endosseous Dental Implant: Assessment of Stability up to 18 Months With the Periotest

Osseointegration is an ongoing histometric process that may vary during clinical function. The implant must be stable at uncovering, which reflects the status of bone-implant interface. The physiology of bone healing associated with endosseous implants suggests that this process occurs between 8 and 12 months, and Periotest values (PTVs) tend to reflect changes in the stability of the bone-implant interface. Stability generally increases gradually from the time of uncovering to an optimal PTV that occurs at a point close to 12 months. This stable interface must remain intact for long-term clinical survival. Rapid development of this optimal PTV is highly desirable in order to prevent premature overloading of the bone-implant interface. The Ankylos implant is a new screw-type implant design in which the thread pitch and length vary to maximize trabecular bone contact. The purpose of this report is to evaluate to 18 months the stability (PTVs) of this implant design. More than 457 implants were placed and followed for a period of 18 months by the multicentered, multidisciplinary Ankylos Implant Clinical Research Group (AICRG). Implant stability (PTVs) was assessed using the Periotest at abutment connection and at 3, 6, 9, 12, and 18 months after uncovering. The Periotest values for all implants rapidly reached an optimal status between uncovering (-3.1 PTVs) and 3 months (-3.4 PTVs). This rapid increase in stability has not previously been reported for other implant designs. The mandibular arch was more negative (-3.8 PTVs) at uncovering as compared with the PTVs for the maxillary arch (-1.7 PTVs). Negative PTVs were recorded (1) as length and diameter increased, (2) as bone density increased, (3) in certain jaw regions, (4) as the number of implants/case increased, and (5) for implants stabile at placement. The Ankylos screw implant design produced rapid stabilization 3 months after uncovering.

Finite element analysis of bone stress and strain around a distal osseointegrated implant for prosthetic limb attachment.

Direct skeletal attachment techniques have recently been identified as an alternative method for percutaneous attachment of prosthetic limbs. Osseointegrated implants for prosthetic attachments are subjected to a complex load condition. This finite element study investigates the effect of varying geometries of the implant on the stress and strain distribution in the area of the bone/implant interface. Simplified three-dimensional axisymmetric models of the femur and seven implants with different core diameters, external diameters, implant lengths and thread pitches were derived. The resulting stress and strain distributions were compared. The significance of each implant geometry was identified for improving implant design in the light of benefit to the bone/implant osseointegration and the bone remodelling of the femur.

A case study with a novel geometry for pulsed laser deposition

Aluminium thin films were deposited by a laser ablation technique from solid cylindrical targets rotated on a lead screw. Both smooth-surfaced targets and targets with screw threads cut into the area to be irradiated were used. The targets were irradiated with a focused circular spot size of 2.5 mm in diameter, at a power density equal to 5.4±0.2 × 108 W cm-2. The polar distribution of the ablated atomic material was found to vary as a function of the target screw thread pitch, with the exponent n, in the polar distribution f() = cosn, varying from 13.5±1.3 for no pitch to a minimum of 5±0.7 for a screw thread pitch of 0.2 mm. The use of such novel target geometries forms a possible basis for increasing thickness uniformity during the pulsed laser deposition of thin films.

Effect of thread pitch on the fatigue strength of steel bolts

The paper explores the effect of the thread pitch on the fatigue strength of ISO steel bolts fitted with standard nuts. By means of boundary element analyses and cross-comparison with theory and photoelasticity, the load capacity of the bolt is ultimately related to a comprehensive notch factor which embodies both stress concentration and notch sensitivity dependencies. The notch factor is conveniently plotted as a function of nominal diameter and thread pitch for a selection of steel grades. The charts show that the endurance load slightly increases as the pitch is decreased for small bolts of low-grade steel. Conversely, the endurance load markedly increases with the pitch (improvements above 40 per cent can be achieved) for large bolts of high-grade steel.

Load Distribution Between Threads in Threaded Connections

Threaded connections are commonly employed in axial load-bearing equipment and pressure vessel components. There are a number of parameters that affect the load distribution between the threads and the stress concentration at the thread roots. These include the thread form, the thickness of walls supporting the threads, the pitch of threads, number of threads engaged, and the boundary conditions. In this paper, the influence of these parameters on the load distribution between threads is reported. Load distribution analyses in threaded connections is performed by analytical and by finite element methods. Square and buttress-type threads have been considered. Three-dimensional nonlinear finite element analyses on threaded connections have been performed using MSC/NASTRAN finite element code. The effect of clearance between the nonmating faces of threads as well as the presence of a flexible media between the mating faces of threads are investigated.

Factors affecting the pullout strength of cancellous bone screws.

Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.

チタニウム製骨結合型，骨接触型骨内インプラントと周囲骨の応力分布に関する研究 第４報 スレッドピッチが周囲骨内応力に及ぼす影響

チタニウム製骨結合型，骨接触型骨内インプラントと周囲骨の応力分布に関する研究 第４報 スレッドピッチが周囲骨内応力に及ぼす影響

Geometric comparison of five interchangeable implant prosthetic retaining screws

Eight geometric parameters of five interchangeable prosthetic retaining screws (#1-3i Implant Innovation-gold, #2-Impla-Med-gold, #3-Nobelpharma-gold, #4-3i Implant Innovation-titanium, and #5-Implant Support Systems-titanium) were recorded with an Amray 1000-B scanning electron microscope at x20 to x200 magnification. Five screws of each type were measured and eight parameters were evaluated: (A) diameter of head, (B) screw length, (C) thread pitch, (D) major diameter, (E) neck diameter (F) length of neck, (G) crest width, and (H) root width. The Nobelpharma-gold prosthetic retaining screws served as controls. The results revealed significant differences between the control and test screws in all parameters except parameters C and G (ANOVA, p < 0.05) and Duncan's multiple range test (significance level 0.05). On the basis of these differences, it was concluded that interchanging prosthetic retaining screws can introduce unknown variables in treating patients.

Effect of Thread Pitch and Frictional Coefficient on the Stress Concentration in Metric Nut-Bolt Connections

Abstract By means of the finite element method, this paper establishes how much the stress state within standard metric nut-bolt connections is affected by variations of the thread pitch and of the frictional coefficient. Following a validated simplified approach, the actual three-dimensional geometry of each connection is replaced by an axisymmetric model which recreates the outline of the joint on an appropriate meridional section. The numerical data prove that, for prescribed nominal thread diameter and bolt load, the peak stress in the screw monotonically increases as the pitch decreases. Further, as far as complete sticking between nut and bolt is not achieved, the stress level linearly increases with the coefficient of friction, the rate of variations being higher at the lowest pitches.