Identifying Screw Threads

  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 readily identify most thread forms. To accomplish this, the features in the list above must be determined.

Frequently, it is necessary to identify threaded parts in the field in order to properly select replacement parts. 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. If the thread size is uncommon, Roton can help users determine which standard size would best replace it.

1. Thread Form back to top
There 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.

FIGURE 43

Thread Form

2. Thread Pitch back to top
The 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.

FIGURE 44

Thread Pitch

3. Major Diameter back to top
The 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 over the least used section of the screw.

FIGURE 45

Major Diameter

4. Pitch Diameter back to top
The 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.

FIGURE 46

Pitch Diameter

5. Minor Diameter back to top
The 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.

FIGURE 47

Minor Diameter

6. Hand of the Thread back to top
The 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.

FIGURE 48

Hand of the Thread

7. Number of Starts back to top
The 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 screw threads 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

Number of Starts

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