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Application Engineering

Useful Formulas

Identifying Screw Threads

Glossary of Terms

Frequently Asked Questions

Engineering Bulletins

Identifying ScrewThreads

1. Thread Form

2. Thread Pitch

3. Major Diameter

4. Pitch Diameter

5. Minor Diameter

6. Hand of the Thread

7. Number of Starts

With the information provided in this section, users can readilyidentify most thread forms. To accomplish this, the features inthe list above must be determined.

Frequently, it is necessary to identify threaded parts in the fieldin order to properly select replacement parts. This can bedifficult for those inexperienced with threads. The large varietyof standard and special thread forms in use along with theincreasing use of metric forms makes the identification taskeven more complicated. After the screw thread is properlyidentified, users can easily select a matching screw and nutfrom Roton's large inventory. If the thread size is uncommon,Roton can help users determine which standard size would bestreplace it.

1. Thread Form back to topThere are many different thread forms in use today. The formsmost widely used for power transmission screw threads areillustrated in Figure 43. An optical comparator is the easiestmethod of determining thread form. Profile gages, if available,and visual methods can also be used. Great care must betaken as many forms are almost identical. The Acme form (29degree included angle) is only 1 degree different from the ISOMetric Trapezoidal form (30 degree included angle). Many

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thread forms such as Unified, Metric ISO and Acme are subjectto published standards while others, including Ballscrew andWorm threads, are not defined in detail by any standardsorganizations.

FIGURE 43

2. Thread Pitch back to topThe thread pitch can be measured with a steel rule, asillustrated in Figure 44, or a caliper or comparator can be used.The thread pitch is the axial distance from one thread grooveto the next. By laying a steel rule down the axis of a screw andcounting the number of thread crests in a given length, thepitch 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 threadsper inch is the reciprocal of the thread pitch. A commonmistake 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 pitchdetermination, check your pitch determined by count againstyour actual pitch measurement.

FIGURE 44

3. Major Diameter back to topThe major diameter can be measured with a micrometer,caliper or steel rule. Major diameters are generally the firstnumbers found in thread designations. A 1/2-10 Acme threadfor example, has a major diameter of .500 in. Care must betaken to measure the major diameter on a section of the screwthread that is not worn. A worn portion will measure smaller

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(or larger if burrs have been rolled up) than the original majordiameter. Therefore, it is good practice to measure the majordiameter over the least used section of the screw.

FIGURE 45

4. Pitch Diameter back to topThe pitch diameter is the diameter at which the thread toothand the thread space are equal. To accurately measure thepitch diameter requires an optical comparator or thread wires.The optical comparator is the easiest to use as themeasurement can be directly made and no mathematics arenecessary. The disadvantage to the optical method is that thescrew must be physically removed from the machine andtaken to the comparator. Also, many small shops may not beequipped with a comparator. Measurement over thread wires isan attractive alternative to the comparator for measuring pitchdiameter. These measurements can be made directly on thescrew. Thread wire measurements are quite accurate,however, they require the use of mathematical formulas alongwith thread form and pitch information to translate themeasurement results into the pitch diameter. Themathematical formula can be found in the Screw ThreadStandards for Federal Services Handbook H28 or otherengineering handbooks dealing with threads. Roton'sapplication engineers can help you with a library of computersoftware which does all the complex calculations in seconds.Contact our application engineers for more details and on-lineassistance with wire measurements and calculations.

FIGURE 46

5. Minor Diameter back to topThe minor diameter can be determined by direct measurementon an optical comparator or by measuring the depth of thethread with a depth micrometer and subtracting twice themeasured depth of thread from the major diameter. Whenusing a comparator to measure the minor diameter, rememberthat the reflected image is reversed (except on modern, image

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correcting comparators). This means that the bottom of theshaft is shown at the top of the screen. Often oil from the shaftruns down and collects on the bottom of the thread groovesincreasing the shadow image. If the oil is not removed, a false(oversized minor diameter) reading will result.

FIGURE 47

6. Hand of the Thread back to topThe hand of the thread can be easily determined by visualinspection. Simply compare your screw threads with the righthand and left hand threads illustrated in Figure 48. Mostthreads are right hand and right hand is assumed if no lefthand designation is specified. Left hand threads are commonon 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 thethread grooves to determine hand. Matching the angle of lie ofthe wire with the illustrations will indicate the hand of thread.

FIGURE 48

7. Number of Starts back to topThe number of starts on most threads is one (single start).However, a number of thread series including Roton's Hi -lead(r), Torqspline(r) Lead Screw, and Ball Screw series screwthreads may have from 2 to 20 starts or more. Multiple startsare used to increase the lead (linear advancement perrevolution). In most cases, increasing the number of starts ispreferable to increasing the pitch because larger pitchesreduce the minor diameter. A small minor diameter decreasesthe screw stiffness and makes it more difficult to tap nutsbecause of the likelihood of the tap breaking during tapping.Also, for the same lead, increasing the number of startsactually increases the thread contact area when compared to athread with the same lead but using fewer starts and a coarserpitch. Close examination of the thread will reveal the numberof starts (Figure 49). Simply place a pencil or marker pen inthe thread groove and rotate the thread one revolution. If the

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end of the pencil mark is in the adjacent thread groove, thescrew has a single start. If there is one thread between thebeginning and the end of the mark, it is a two start thread, twogrooves, a three start thread and so on. Another way todiscover the thread starts is to examine a transverse section ofthe screw. As illustrated in Figure 49, if the end view is anoffset circle, the screw is single start. A two start thread willhave roughly a football shape, a three start thread will have atri-oval shape and a four start thread will be noticeably fourcornered. Usually, five starts and up can simply be counted inthe transverse section.

FIGURE 49

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