Inspecting a Tr trapezoidal thread
On the drawing of a load-carrying lead screw for a hydraulic press, the thread is specified as Tr 36 x 6 - 7e. The turner has to validate the 30° flank width and the pitch diameter d2 using the calibrated wire method before sending the part for heat treatment.
Formulas and method for inspecting a Tr trapezoidal thread
| Tr thread characteristic | Formula for a 30° angle | Example Tr 36 x 6 (mm) |
|---|---|---|
| Nominal pitch diameter d2 | d2 = d - 0.5 * P | 36 - 3 = 33.000 mm |
| Best wire size W | W = 0.5176 * P | 0.5176 * 6 = 3.106 mm |
| Measurement over wires M | M = d2 + W * (1 + 1/sin(15°)) - 0.5 * P * cot(15°) | M = d2 + 4.8637 * W - 1.866 * P |
All values are given for the reference environmental conditions of 20 °C.
Calculating the measurement over wires M for a 30° trapezoidal profile
Unlike the 60° metric thread, the trapezoidal thread has a 30° profile angle (15° half angle). The formula for the measurement over wires is M = d2 + 4.8637 * W - 1.866 * P. To compare with standard metric threads, read our guide on inspecting an ISO metric thread with wires and gauges.
Class 7e tolerances to ISO 2903
Position e gives the pitch diameter a negative upper deviation es, to guarantee running clearance under load.
Worked example: inspecting a Tr 36 x 6 screw
Screw Tr 36 x 6 - 7e (d2 max = 32.850 mm, d2 min = 32.580 mm). Wires W = 3.106 mm. Reading over wires M = 36.780 mm. 1. Calculated d2 = 36.780 - 4.8637 * 3.106 + 1.866 * 6 = 32.870 mm. Verdict: the calculated d2 (32.870 mm) exceeds the upper limit of 32.850 mm. The thread is too full.
Practical pitfalls when inspecting Tr threads
The first pitfall is using wires meant for 60° threads instead of 30° wires. The second is poor alignment of the wires in the coarse pitch. The third comes from burrs at the thread root. To manage your special thread inspections, see our inspection module.