Dear MISUMI Engineering Team:
We are currently building a machine that has several timing shafts that are driven by spur gears, but are having problems determining the correct spacing between the shafts. The shafts are held by bearings mounted on a fixture. And to follow up, once we know the correct center to center shaft spacing to use, how much of a tolerance do we need to hold on that measurement?
Stacy, thanks for your questions. Before we get to answering them, we should go over a little background information for other readers. If you look at the featured picture, you will see an example of two spur gears with keyways that could be mounted on the end of two shafts. The gears are sitting in a “meshed” state, meaning that the teeth of the gears are engaged with each other and that they are ready to transmit power. While it may look like all toothed gears are created equal, there are several choices for the “pressure angle” of the teeth, traditionally 14.5°, 20°, and 25°. This is the angle at which the two opposing teeth meet when meshing. A larger angle means larger teeth and therefore more strength and torque handling capacity. However, a smaller angle means less backlash, a higher contact ratio, and smoother, quieter operation. Regardless of which angle you choose, all meshing gears must have the same angle in order to function properly!
Now, we are going to look at Stacy’s question about shaft distance. If you look at the diagram above, you will see Center Distance (C) listed on the right side. Using a tolerance of H7, the correct center distance will be when the pitch diameters (labeled pitch circle above) of both gears are touching each other. Generally, the pitch diameter for a gear is supplied by the manufacturer, but if you need to calculate it:
The center distance would then be:
If you have questions about those calculations, or want more information about spur gears, this site is a great resource.
Due to the way spur gears are manufactured, using this H7 tolerance on the center distance will produce an average backlash between the gears. Backlash is the clearance or lost motion caused by gaps between the gears. A small amount of backlash is necessary to prevent binding, but too much can cause excessive noise or positioning errors in the shafts. Knowing your required backlash is important because it drives the tolerance on the center distance. Regardless of the desired backlash or the type and size of the gears being used, extreme caution should be taken when specifying a negative tolerance. If the shafts are placed too close together, it can cause interference and gear binding, which can lead to premature wear or catastrophic failure. For an average application, fine pitched gears could have a +0.002 inch center distance tolerance and coarse pitched gears could go up to +.005 inches.
Finally, be careful of indiscriminately specifying a low center distance tolerance. While it looks good on paper, very tight machining tolerances can be expensive and the same effect might be achieved by using a higher tolerance gear. As always, if your application requires a specific backlash or has other critical factors, it is best to double-check your tolerances with our Engineering Team at firstname.lastname@example.org.