The thing with gears is that if they slip off the shaft it’s usually not because some part broke, but because you picked wrong size in the first place. That’s what happens when you pick your key by guessing at it instead of using geometry. A Woodruff key is just a plain old disc of steel and looks like any other one until torque rips it right off the shaft. The diagram above shows exactly how big it needs to be so it doesn’t do this. It makes sense out of all those confusing part number.
What Woodruff keys do and how to use them The primary benefit is that they finds their own place. With standard square keys, you must have exactly axial alignment before sliding on the hub. Woodruff keys has a curved seat milled out of the shaft and drop right in without any adjustment. They self-center automaticly. That’s why it’s a popular key type for smaller electric motor or even automotive camshafts with tapered shafts. The curve allow for some very slight angular misalignment without binding two parts together.
How to Use Woodruff Keys Correctly
Simply select the proper size for your application. The secret to making the right cut starts by learning how to read the number code on keys. As the chart below explains, each number correspond to the diameter and width of the key. To make it easy to remember, the left-hand numbers is the width in thirty-seconds of an inch; the final pair of numbers correspond to the diameter in eighths of an inch. So if the number on a key is 608, then the key measure one inch in diameter and three-sixteenths of an inch wide. This coding standard ensure the number on the key matches the number on the cutter used to mill the seat. No conversion math required at the mill, just pull out cutter 608 for key 608.
More than most people in this hobby realize, material selection are important. For general machined parts used in non-corrosive settings, cold rolled steel (such as 1018) is sufficient for light load application. If you’re going to have high torque demands or shock loads, consider using heat treated alloy steels (such as 4140). These materials has better fatigue resistances and shear strengths. In food processing applications or marine applications, stainless steel is the clear winner though it is a bit less strong. Do you want torque or do you want rust? It’s up to you when making your design decision.
Structural considerations do put limits on these keys. Because of the curvature of the seat, they remove more material then a shallow square keyway would, which creates more stress concentration in the shaft. That reduces the shaft’s structural integrity, limiting Woodruff keys to typically smaller diameters, most commonly less than an inch and a quarter. Beyond that size, there is not enough of the shaft left to resist the forces without risking a fracture. Physics determines this limit, not personal preference. Parallel keys or splines replaces Woodruff keys beyond this point.
“Measure the shaft correctly and start there on selecting the proper size. A 204 or 304 key works well for a quarter-inch shaft, according to the chart. Then a 507 or 607 would of be a good choice for a half-inch shaft since it’s going to fill out the shape of the shaft. Always check the rest of the wall thickness before you cut the seat. You don’t want to make it too deep compared to the shaft size. If you do, you create a weakness that cannot be fixed with lubrication. What you are looking for is something tight that will transfer torque but not strip out the metal.”
The trick to proper use of a Woodruff key is understanding that it’s a load bearing connection between two rotating part; its shape matter. Match the key to your cutter, get the correct depth on the shaft within normal tolerances and it’ll stay put. You won’t see it at work in the hub bore because it’s designed to self center…quietly. However you WILL notice the result: a machine running true, staying in line under load. Simple equals precise…simple isn’t always unimportant. Many people think that.
