ELECTRICAL REBUILDER’S ASSOCIATION—Technical Document CS201407D www.electricalrebuilders.org Understanding Root Diameter Does it matter? BY BOB THOMAS AND WES GRUENINGER SR. Figure 1—Nomenclature of gear teeth. f you look up any starter drive in a catalog, a reference book or an online database, you will probably find specifications like the number of teeth, the direction of rotation, the number of splines, the drive length, the gear OD TOOTH HEIGHT (outside diameter), the gear ID (inside diameter) and possibly the drive’s OD at the widest point. Root Diameter (RD) is seldom ever mentioned, except to differentiate between two nearly identical drives. What is the RD, and does it really matter? Will drives that differ only by RD interchange with one another? Pinion OD is the cross-sectional measurement of the gear taken across a circle drawn around the outside of the tops of the gear’s teeth. Pinion RD is the cross-sectional measurement taken across a circle drawn around the base of the gear’s teeth. The height of the gear teeth accounts for the difference between the two diameters. If you think of the gear teeth as a series of mountain tops the root will be the bottom of the valleys between them. The root is also often called the base of the gear. Measuring the root diameter with an even number of teeth is relatively easy (see Figure 2). When working with an odd number of teeth you will have Figure 2— Measuring RD of a drive pinion with an even number of teeth is relatively easy using calipers. Figure 3— Measuring RD when the pinion has an odd number of teeth requires making a careful estimate of the root circle. An ERA Publication. ©2014, Electrical Rebuilder’s Association. All rights reserved. No portion of the guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consent of the Electrical Rebuilder’s Association. Page 1 ELECTRICAL REBUILDER’S ASSOCIATION—Technical Document CS201407D www.electricalrebuilders.org to estimate, but can still get close (see Figure 3 on page 13). To understand when and how RD can be important, you have to understand why you need clearance between two meshing gears. For any set of gears to operate efficiently with the least amount of wear and loss of energy due to friction, there must be some amount of clearance between them. In other words, they cannot be meshed tightly together with zero tolerance. At the same time, they cannot have too much clearance either. Insufficient clearance increases friction, causing rapid wear and a loss of torque. In the case of a starter drive pinion, it will also cause excessive pressure and wear to the support bushing or bearing in the starter’s drive end housing. Insufficient clearance between a starter drive pinion and a ring gear can even cause the gears to bind during cranking. When that happens, the return spring on the plunger may not be able to retract the pinion quickly, once the engine starts. A good example of this and the symptoms it might cause are explained in detail in part two of Dan Marinucci’s Motor Magazine article, reprinted on page 8. If, on the other hand, clearance between a pinion and a flywheel is excessive, it will shift the contact or working surfaces of both gears toward the tops of the teeth, causing them to wear more quickly. Excessive clearance, if extreme, may allow the pinion to skip over the teeth on the flywheel under pressure, eventually ruining both gears. We will address excessive clearance problems next month. Simply knowing the RD of a pinion gear may not help you much in determining clearance for a particular problem vehicle. Few applications provide access to take actual measurements of pinion to ring gear clearance once the starter is installed. Most of the time, “witness” (or wear) marks on the pinion gear are your only guide. Even if a pinion has cranked an engine only a few times, you can usually see the depth to which the pinion and flywheel were mating. Those wear marks are your best evidence of whether or not the pinion was working with the correct amount of clearance (see Figures 4 and 5 on page 13). Whenever you tear down or test any starter, pay particular attention to the pinion gear, and do not ignore the root area. A used pinion can tell you a lot if you examine it closely. Pinions may not be able to speak, but they still have a story to tell. Next month we will explain the signs of excessive clearance and some of the circumstances which can cause it. Figure 4— The wear marks on this pinion reveal the surfaces of the gear that made contact with the ring gear. Notice the root area of this drive was never touched, indicating there was clearance between it and the ring gear on the flywheel. Figure 5— The wear marks on this pinion indicate there was insufficient clearance. The root area is just as shiny as the working surfaces. Those who recognize this popular drive probably know the application and remember the problems it caused in the 1980s. We will cover that in detail next month. An ERA Publication. ©2014, Electrical Rebuilder’s Association. All rights reserved. No portion of the guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consent of the Electrical Rebuilder’s Association. Page 2