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March 30, 2021

locomotive traction motor pinion EMD 9556211

How the improvement in the design of a traction motor pinion by SUPCO will prolong the pinion life span and reduce the maintenance costs of different types of traction motors.
When it comes to traction motor pinion gears, there is always a slight sliding or slipping on both sides of pitch point over tooth curvature. This sliding causes pitting and damages the traction motor pinion teeth, which as a consequence produces torsional vibration when rotating the gearwheel, and damaging the traction motor. In this paper, SUPCO demonstrates the root cause of such damages and proposes the method for how to improve the traction motor pinions. Contact SUPCO for more information on traction motors.

An Introduction to Locomotive traction motor Pinion :

Transmission of rotational force from one axle to the other has a long history by using pulley and belt. This is still in use for many applications. However, when there is a high amplitude of force then due to tangential traction between belt and pulley, and limited fraction counterforce, slipping between locomotive traction motor pinion gear will be the main issue. Clip 1. Therefore, an alternative means of transmission such as spur gear is to be utilized.

As the case is in locomotives, where the traction motors are driving the locomotive’s axle through axle-gears. Spur gear transmission transfers force from locomotive traction motor’s pinion to axle gear as Normal force to the gear’s teeth compare to tangential force between pully and belt, so direct push will be applied by meshing 2 gears. See Clip 2

Replacing pullies by meshing gears, and string by engaging pinion and axle gear’s teeth. One should study the correct section of engaging teeth to have a constant velocity ratio while these 2 teeth are touching and passing one over another, see Fig 3, and video clip 2

traction motor pinion Section

The experiment on video clip 5 shows how such curve called as involute can be generated. To trace this curve, we locate a pen at point P on this string to draw the tooth profile for each pulley. The planes where these two profiles are drawing on, is bond to the surface of their own pulley, as a rigid part turning together. For better understanding the 2 profiles generated, here on this clip are shown again on Fig 8 of the next.

Chalanges in designing Traction Motor Pinion Gears

Since both Involute sections are generated simultaneously at common point P, they constitute the shape of the 2 engaging tooth, having contact at pitch point P. This is the point where the tooth on driving gear or pinion is touching the opposite tooth on driven gear installed over locomotive’s axle. Within a short period of time, when the 2 opposite teeth are meshed, this touching or pitch point is tracing these 2 wedding profiles.

In fact the touching point P is

traction motor pinion Section view
Fig 6

moving along 3 different paths depending on the plane in which the pen on point p is sketching on, Fig 6.

1)- Lower involute generated by red pulley .

2)- Upper involute generated by Green pulley.

3)- The straight line tangent to both pulleys is called line of action or pressure line .

Clip 5 on previous page , shows how these profiles are generating

The 2 wedding involutes shown on Fig 6 always make a contact at point P, a direct push is applied and a steady motion with constant velocity is transferred. As mentioned earlier point P is moving with a constant speed along Pressure Line tangent to the base circle of both gears, exactly as it is the case in pulleys and belt mechanism, so the velocity ratio of these 2 gears is equal to the ratio of their Base Circle Diameters. Although the contact point is moving with constant speed along Pressure Line, but since it moves on different speeds over the wedding involutes, therefore it is our main concern as the root cause of gears wear.

At the first stage, the involute generated by axle gear growing faster before reaching pitch point C (See Fig 6, and Fig 7 A ) so the slides passing the second involute generated by traction motor pinion gear, at the single point where the tangent point of the 2 Pitch circles, these two involutes have the same speed, so there is no sliding. Then at the third stage, after passing the pitch point, the involute generated by pinion grows faster and slides passing over the involute produced by axle gear. So there is always sliding in both sides of the pitch point. Consequently, the optimum tooth curve is the small segment at 2 sides of the pitch point, called Addendum and Dedendum sections. Even small, but sliding always happens in 2 opposite directions around pitch point in locomotive traction motor pinion gear, during tooth engagement.


SUPCO is a company that is producing and exporting Traction Motor Pinion Gears. It includes experts from different branches of Locomotive designing to establish a potential atmosphere for competition in order to create the most qualified Locomotive Traction Motors. Our company is a highly dependable compound for the best North American producers, which offers a long-term relationship and gives after-sale services to customers. To see our products, more information on traction motor pinion visit the Traction Motor Products page on our website. Also, check other articles on our blog for different types of traction motors and details of AC vs DC traction motors.

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Posted by      Isobel Flynn at 4:25 AM MDT


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