A trapezoidal thread spindle is an element used for manual and motor drive. The trapezoidal screw transmits rotating movements into linear movements. Characteristic of the trapezoidal screw is the name-giving, isosceles and trapezoidal thread profile on the spindle. The flank angle of a trapezoidal thread is 30° and is standardised according to DIN 103 and ISO 2901/2903. The individual threads of a trapezoidal thread have a thicker profile than triangular threads, which are common with metric or Whitworth threads, among others. The pitch of a trapezoidal thread is also significantly larger..
Drives with trapezoidal thread spindles can be manufactured at low cost and are correspondingly widespread. The spindles are used to convert rotary into linear motion. For this purpose, the spindle is set in rotation by a motor and drives a threaded nut via the threads, which moves linearly on the trapezoidal threaded spindle. The direction of rotation of the spindle determines the direction of movement of the spindle nut.
Trapezoidal threaded spindles are always used when the factor of self-locking is rated higher than the speed and precision of spindle adjustment. Depending on the material pairing, i.e. the combination of materials of shaft and nut, there are different areas of application for trapezoidal thread spindles. The spindles are used, among other things, in vice and machine guides, are used as a component in assembly blocks or within an adjustment strip on a guide..
Trapezoidal thread spindles are generally manufactured by thread rolling, which makes the production process inexpensive and therefore extremely economical. Due to thread rolling, trapezoidal thread spindles are very wear-resistant, have a high tensile strength and are extremely resistant to bending. As there is always sliding friction between the thread flanks and the thread nuts in a trapezoidal thread, the use of the spindles is naturally limited to adjustment processes which are not too fast and which remain static for a longer period of time after adjustment. Due to the large pitch, a trapezoidal thread covers more distance per revolution of the spindle than metric threads. Furthermore, trapezoidal threads are insensitive to contamination by dust or sand.
When selecting the appropriate trapezoidal thread spindle, you inevitably come across at first cryptic-looking abbreviations like "TR8x1.5". This thread designation, standardised by the way, consists of three elements on closer inspection:
A thread TR8x15.5 is therefore a trapezoidal thread with a nominal diameter of 8 millimetres and a pitch of 1.5 mm
These days, fortunately, trapezoidal threads are all standardised - unless you have caught a real exotic from the Far East! If you have found a spindle with unknown data, the pitch can of course be measured without any problems. In professional production, so-called gauge rings are used for this purpose.
Also used here are flank micrometers and in rare cases optical measuring instruments. However, the pitch of a trapezoidal thread can be determined more easily and quickly with a caliper gauge. In the ISO system, the pitch refers to the distance between two thread tips (i.e. the highest points of a thread on the shaft).
Here, the measurement is then simply taken from "centre to centre" using a caliper gauge - the value displayed corresponds to the pitch. The situation is different with inch threads. Here the pitch is defined as the number of thread tips per inch. A little bit of conversion work is needed here!