.Identifying Screw Threads. With the information provided in this section, users can readily identify most thread forms. To accomplish this, the features in the list below must be determined.Frequently, it is necessary to identify threaded parts in the field in order to properly select replacement parts and to choose the right type of screw. This can be difficult for those inexperienced with threads. The large variety of standard and special thread forms in use along with the increasing use of metric forms makes the identification task even more complicated. After the screw thread is properly identified, users can easily select a matching screw and nut from Roton’s large inventory.
![Screw Thread Depth Formula Screw Thread Depth Formula](/uploads/1/2/5/5/125510572/407183330.gif)
If the thread size is uncommon, Roton can help users determine which standard size would best replace it. See our guide below in how to measure a screw and its thread. Thread FormThere are many different thread forms in use today. The forms most widely used for power transmission screw threads are illustrated in Figure 43. An optical comparator is the easiest method of determining thread form.
Profile gages, if available, and visual methods can also be used. Great care must be taken as many forms are almost identical. The Acme form (29 degree included angle) is only 1 degree different from the ISO Metric Trapezoidal form (30 degree included angle). Many thread forms such as Unified, Metric ISO and Acme are subject to published standards while others, including Ballscrew and Worm threads, are not defined in detail by any standards organizations. Thread PitchThe thread pitch can be measured with a steel rule, as illustrated in Figure 44, or a caliper or comparator can be used. The thread pitch is the axial distance from one thread groove to the next. By laying a steel rule down the axis of a screw and counting the number of thread crests in a given length, the pitch can be determined by dividing the count into the length.
In the example shown (Figure 44), there are 5 pitches in 1 in. So the thread pitch is.200 in. Note that the number of threads per inch is the reciprocal of the thread pitch.
A common mistake is to count the number of threads starting with “one”. This will lead to a one pitch error. Make sure you start with “zero” for the first thread. To double check your pitch determination, check your pitch determined by count against your actual pitch measurement. Major DiameterThe major diameter can be measured with a micrometer, caliper or steel rule.
Major diameters are generally the first numbers found in thread designations. A 1/2-10 Acme thread for example, has a major diameter of.500 in. Care must be taken to measure the major diameter on a section of the screw thread that is not worn. A worn portion will measure smaller (or larger if burrs have been rolled up) than the original major diameter. Therefore, it is good practice to measure the major diameter and screw size over the least used section of the screw.
Part 1: Thread relief depths.613 / n =.005 or.613 divided by the number of threads per inch +.005 thousanths = the depth to plunge the grooving tool into the thread diameter to make a thread relief (based on a.001 move on the crossfeed dial makes a.001 move into the part, or removes.002 on the diameter). The screw should fail before the thread strips. For this, it is necessary the shear area of the threaded feature be at least 2 times the tensile area. (Assuming the threaded feature is of the identical material). Where: L e = Minimum Thread Engagement Length A t = Tensile Stress Area D = Major Diameter of fastener (screw).
Pitch DiameterThe pitch diameter is the diameter at which the thread tooth and the thread space are equal. To accurately measure the pitch diameter requires an optical comparator or thread wires. The optical comparator is the easiest to use as the measurement can be directly made and no mathematics are necessary. The disadvantage to the optical method is that the screw must be physically removed from the machine and taken to the comparator.
Also, many small shops may not be equipped with a comparator. Measurement over thread wires is an attractive alternative to the comparator for measuring pitch diameter. These measurements can be made directly on the screw. Thread wire measurements are quite accurate, however, they require the use of mathematical formulas along with thread form and pitch information to translate the measurement results into the pitch diameter.
The mathematical formula can be found in the Screw Thread Standards for Federal Services Handbook H28 or other engineering handbooks dealing with threads. Roton’s application engineers can help you with a library of computer software which does all the complex calculations in seconds. Contact our application engineers for more details and on-line assistance with wire measurements and calculations. Minor DiameterThe minor diameter can be determined by direct measurement on an optical comparator or by measuring the depth of the thread with a depth micrometer and subtracting twice the measured depth of thread from the major diameter. When using a comparator to measure the minor diameter, remember that the reflected image is reversed (except on modern, image correcting comparators). This means that the bottom of the shaft is shown at the top of the screen.
Often oil from the shaft runs down and collects on the bottom of the thread grooves increasing the shadow image. If the oil is not removed, a false (oversized minor diameter) reading will result. Hand of the ThreadThe hand of the thread can be easily determined by visual inspection. Simply compare your screw threads with the right hand and left hand threads illustrated in Figure 48. Most threads are right hand and right hand is assumed if no left hand designation is specified. Left hand threads are common on manual drives where clockwise handle rotation raises, tightens, extends, or creates motion away from the operator. On fine threads, it may be necessary to lay a small wire in the thread grooves to determine hand.
Matching the angle of lie of the wire with the illustrations will indicate the hand of thread. Number of StartsThe number of starts on most threads is one (single start). However, a number of thread series including Roton’s, and may have from 2 to 20 starts or more. Multiple starts are used to increase the lead (linear advancement per revolution). In most cases, increasing the number of starts is preferable to increasing the pitch because larger pitches reduce the minor diameter.
A small minor diameter decreases the screw stiffness and makes it more difficult to tap nuts because of the likelihood of the tap breaking during tapping. Also, for the same lead, increasing the number of starts actually increases the thread contact area when compared to a thread with the same lead but using fewer starts and a coarser pitch. Close examination of the thread will reveal the number of starts (Figure 49). Simply place a pencil or marker pen in the thread groove and rotate the thread one revolution. If the end of the pencil mark is in the adjacent thread groove, the screw has a single start. If there is one thread between the beginning and the end of the mark, it is a two start thread, two grooves, a three start thread and so on. Another way to discover the thread starts is to examine a transverse section of the screw.
As illustrated in Figure 49, if the end view is an offset circle, the screw is single start. A two start thread will have roughly a football shape, a three start thread will have a tri-oval shape and a four start thread will be noticeably four cornered. Usually, five starts and up can simply be counted in the transverse section.Figure #49.Product Filter.
The thread form is the configuration of the thread in an axial plane; or more simply, it is the profile of the thread, composed of the crest, root, and flanks. At the top of the threads are the crests, at the bottom the roots, and joining them are the flanks. The triangle formed when the thread profile is extended to a point at both crests and roots, is the fundamental triangle. The height of the fundamental triangle is the distance, radially measured, between sharp crest and sharp root diameters. The distance measured parallel to the thread axis, between corresponding points on adjacent threads, is the thread pitch.
Unified screw threads are designated in threads per inch. This is the number of complete threads occurring in one inch of threaded length. Metric thread pitch is designated as the distance between threads (pitch) in millimeters.On an internal thread, the minor diameter occurs at the crests and the major diameter occurs at the roots. On an external thread, the major diameter is at the thread crests, and the minor diameter is at the thread roots. The flank angle is the angle between a flank and the perpendicular thread axis.
Flank angles are sometimes termed 'half-angle' of the thread, but this is only true when neighboring flanks have identical angles; that is, the threads are symmetrical. Unified screw threads have a 30° flank angle and are symmetrical. This is why they are commonly referred to as 60° threads.Pitch diameter is the diameter of a theoretical cylinder that passes through the threads in such a way that the distance between the thread crests and thread roots is equal. In an ideal product, these widths would each equal one-half of the thread pitch.An intentional clearance is created between mating threads when the nut and bolt are manufactured. This clearance is known as the allowance. Having an allowance ensures that when the threads are manufactured there will be a positive space between them.
For fasteners, the allowance is generally applied to the external thread. Tolerances are specified amounts by which dimensions are permitted to vary for convenience of manufacturing. The tolerance is the difference between the maximum and minimum permitted limits. Thread FitThread fit is a combination of allowances and tolerances and a measure of tightness or looseness between them. A clearance fit is one that provides a free running assembly and an interference fit is one that has a positive interference thus requiring tools for the initial run-down of the nut.For Unified inch screw threads there are six standard classes of fit: 1B, 2B, and 3B for internal threads; and 1A, 2A, and 3A for external threads. All are considered clearance fits.
That is, they assemble without interference. The higher the class number, the tighter the fit. The 'A' designates an external thread, and 'B' designates an internal thread. Classes 1A and 1B are considered an extremely loose tolerance thread fit.
![Screw thread depth formula 3 Screw thread depth formula 3](/uploads/1/2/5/5/125510572/214363759.gif)
This class is suited for quick and easy assembly and disassembly. Outside of low-carbon threaded rod or machine screws, this thread fit is rarely specified. Classes 2A and 2B offer optimum thread fit that balances fastener performance, manufacturing, economy, and convenience. Nearly 90% of all commercial and industrial fasteners use this class of thread fit. Classes 3A and 3B are suited for close tolerance fasteners. These fasteners are intended for service where safety is a critical design consideration. This class of fit has restrictive tolerances and no allowance.
Socket products generally have a 3A thread fit. Go and No-Go Gauges are threaded rings that are tapped in such a way that they ensure proper tolerancing of parts.
Similar devices are available for internally threaded fasteners.Per the acceptance requirements of ASME B1.3, System 21, the allowance specified for the Class 2A external threads is used to accommodate the plating thickness. The plain finished parts (or plated parts prior to plating) would be tested for adherence to these tolerances with a 2A Go/No-Go thread gauge. The 2A Go gauge would ensure the pitch diameter falls below the maximum requirement; the No-Go gauge would verify that the pitch diameter is above the minimum requirement.
A standard electro-zinc plated 2A part would be gauged with the Class 3A Go (due to the plating metal thickness) and 2A No-Go gauge after plating.Thread damages such as dents, scrapes, nicks, or gouges and plating build-up are not cause for rejections unless they impair function and usability. Threads that do not freely accept the appropriate Go ring gauge shall be inspected by allowing the screwing of the gauge with maximum allowable torque value of:Torque = 145 x d 3 (for inch series), where Torque is in-lbs. And d is diameter in inches - IFI 166Torque = 0.001 x d 3 (for metric series), where Torque is Nm and d is diameter in mm - IFI 566 Thread SeriesThere are three standard thread series in the Unified Screw Thread System that are highly important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread). A chart listing the standards sizes and thread pitches with their respective thread stress areas is listed in the Fastenal Technical Reference Guide, along with a special series designated UNS.Below are some of the aspects of fine and coarse threads.
Coarse Thread. Stripping strengths are greater for the same length of engagement. Improved fatigue resistance. Less likely to cross thread. Quicker assembly and disassembly. Tap better in brittle materials. Larger thread allowances allow for thicker platings and coatingsNumerous arguments have been made for using either fine or coarse threads; however, with the increase in automated assembly processes, bias towards the coarse thread series has developed.
UNR ThreadsThe UNR thread is a modified version of a standard UN thread. The single difference is a mandatory root radius with limits of 0.108 to 0.144 times the thread pitch. When first introduced decades ago, it was necessary to specify UNR (rounded root) threads. Today, all fasteners that are roll threaded should have a UNR thread because thread rolling dies with rounded crests are now the standard method for manufacturing most threads. UNJ ThreadsUNJ thread is a thread form having root radius limits of 0.150 to 0.180 times the thread pitch. With these enlarged radii, minor diameters of external thread increase and intrude beyond the basic profile of the UN and UNR thread forms.
Consequently, to offset the possibility of interference between mating threads, the minor diameters of the UNJ internal threads had to be increased. 3A/3B thread tolerances are the standard for UNJ threads. UNJ threads are now the standard for aerospace fasteners and have some usage in highly specialized industrial applications.UNJ bolts are like UNR, but the curve of the thread root is gentler which requires that it be shallower.
In fact, the thread root is so shallow that the bolt thread cannot mate with a UN nut, so there is a UNJ nut specification as well. Threads can be produced by either cutting or rolling operations. The shank of a blank designed for cut threading will be full-size from the fillet under the head to the end of the bolt. Producing cut threads involves removing the material from a bolt blank with a cutting die or lathe in order to produce the thread. This interrupts the grain flow of the material.Rolled threads are formed by rolling the reduced diameter (approximately equal to the pitch diameter) portion of the shank between two reciprocating serrated dies.
The dies apply pressure, compressing the minor diameter (thread roots) and forcing that material up to form the major diameter (thread crests). Imagine squeezing a balloon with your hand; you compress with your fingers to form the valley, while allowing part of the balloon to expand between your fingers. This is the concept behind roll threading. Rolled threads have several advantages: more accurate and uniform thread dimension, smoother thread surface, and generally greater thread strength (particularly fatigue and shear strength).Thread cutting requires the least amount of tooling costs.
It is generally only used for large diameter or non-standard externally threaded fasteners. Thread cutting is still the most commonly used method for internal threads.
Thread StrengthTwo fundamentals must be considered when designing a threaded connection. Ensure that the threaded fasteners were manufactured to a current ASTM, ANSI, DIN, ISO or other recognized standard. Ensure that the design promotes bolts to break in tension prior to the female and/or male threads stripping.
A broken bolt is an obvious failure. However, when the threads strip prior to the bolt breaking, the failure may go unnoticed until after the fastener is put in service.Internal Thread Strength FormulaF = Su.
AtsSu = shear strength of the nut or tapped materialAts = cross-sectional area through which the shear occursThe strength of bolts loaded in tension can be easily determined by the ultimate tensile strength. To determine the amount of force required to break a bolt, multiply its ultimate tensile strength by its tensile stress area, As.Determining the strength of the threads is more complicated. Since the male threads pull past the female threads, or vice-versa, the threads fail in shear and not in tension.
Therefore, the stripping strength of an assembly depends on the shear strength of the nut and bolt materials. Taking proper precautions during the design phase is vital to avoiding thread failure.
Once the first engaged thread begins to shear, the threads behind it will also shear in rapid succession.To determine the force required to strip the threads, multiply the shear strength by the cross sectional area being sheared. The difficulty lies in determining the cross sectional area in which the shear will occur.
Here are three possible scenarios for this type of failure. The nut material is stronger than the bolt material. In this example, the nut threads will shear out the bolt threads. The failure will occur at the root of the bolt threads.
The bolt material is stronger than the nut material. In this scenario, the bolt threads will shear out the nut threads. The failure will occur at the root of the nut threads.
The nut and bolt are the same strength. In this scenario, both threads will strip simultaneously. This failure will occur at the pitch line.The tensile strength of most fasteners is usually specified, whereas shear strength is not. In order to avoid shearing the threads, ensure that the length of engagement between the internal and external thread is long enough to provide adequate cross-sectional thread area.Failure scenarios #1 and #3 can typically be avoided by ensuring proper thread engagement. With proper engagement, those scenarios would result in a tensile failure of the bolt rather than thread stripping. Generally the hardness and the actual material strength of a nut is less than the bolt. For example, if you look at the hardness of an SAE J995 Grade 8 nut (HRC 24-32 up to 5/8-in diameter), it is likely to be less than the SAE J429 Grade 8 bolt (HRC 33-39).
This is designed to yield the nut threads to ensure the load is not carried solely by the first thread. As the thread yields, the load is further distributed to the next five threads. Even with the load distribution, the first engaged thread still takes the majority of the load. In a typical 7/8-9 Grade 8 nut, the first engaged thread carries 34% of the load. Using internally threaded materials with higher strengths and hardness can often result in fatigue and/or loosening.The strength capacities of standard nuts are listed as the nut's proof stress. This should not be confused with the proof strength of the bolts.
Proof stress is the ultimate load the nut can support without thread failure. For design purposes, the most important aspect of choosing the appropriate nut is to select a nut with a proof stress equal to or greater than the ultimate tensile strength of the bolt.Caution: It appears that one could theoretically increase the thread strength by increasing the length of engagement. However, as illustrated in the Load Distribution chart above, the first thread will be taking the majority of the applied load. For carbon steel fasteners (including tapped holes) the length of engagement would be limited to approximately one nominal diameter (approximately 1-1/2 times the diameter for aluminum).
After that, there is no appreciable increase in strength. Once the applied load has exceeded the first thread's capacity, it will fail and subsequently cause the remaining threads to fail in succession. If the nut proof stress does not exceed the proof strength of the bolt, stripping failure is very likely.Returning to the discussion of fundamentals in thread connection design, the nut or tapped hole should support more load than the bolt. Thus, the design criteria for threaded connections also leads to nut selection criteria which help the designer ensure functionality in the joint. The following are the basic rules:.
Ensure that the nut adheres to a specification which is compatible with the specification of the bolt (ASTM A193 and ASTM A194, SAE J429 and SAE J995, etc.). Ensure that the selected nut has a proof stress greater than or equal to the tensile strength of the bolt.