Module and DP in RC gears

Whenever you need to replace the pinion or spur gear, you first need to know which module or DP your RC car allows, which are two different classification according to metric or imperial manufacturing.

We want provide a hint of theory and the definitions of Pitch, Module and Diametral Pitch (DP).

Theory of gears

The gears drive each other thanks to the teeth, but the motion is equivalent to 2 respective wheels touching at the point of tangency. The diameters of these wheels are d1 and d2 as shown in the picture.

Pitch

Pitch is the length of the arc of the respective wheel measured between the beginning of one tooth and the next.

Module (M)

The module is used in the metric standard and is the ratio of the respective wheel diameter to the number of teeth.

Module 0.4

Module 0.5

Module 0.6

Module 0.7

Module 0.8

Module 0.9

Module 1

Diametral Pitch (DP)

Diametral Pitch belongs to the imperial standard and is the ratio of one inch (25.4mm) to the Module. DP commercially available:

DP 24

DP 32

DP 48

DP 64

Module and DP comparison

With the information learned so far, we can compare the tooth sizes of pinions and spurs according to their respective manufacturing standards. The table is provided with increasing pitch.

Metric	Pitch (mm)	Imperial
	1,25	DP 64
Module 0.4	1,26
Module 0.5	1,57
	1,66	DP 48
Module 0.6	1,88
Module 0.7	2,2
	2,49	DP 32
Module 0.8	2,51
Module 1.0	3,14
	3,32	DP 24

Conclusions

Knowing the classification in Module and DP is helpful to recognize the gears and avoid installation or even wrong purchases.

You must always comply with the starting standard when replacing a pinion or spur gear, but there are 2 exceptions where the parts can be classified into both standards, and they are M0.4/DP64 and M0.8/DP32; in fact, these sizes have pitch differences of 1 or 2 cents of a millimeter, which are tolerable as they are comparable to manufacturing and assembly tolerances.

Sometimes it is preferred to change the size of the teeth for strength or noise reasons; the change is possible as long as it affects both the spur gear and the pinion.

Good job!