In every one of those machines in the factory, there is a shaft spinning inside a hub. To do its job, that shaft must be able to transfer force and not slip while doing so. Enter the parallel key. It is a little piece of often-overlooked metal that will cause problems if it are removed during maintenance or sheared off under load. But the keys themselves is standardized by DIN 6885 and ISO 773 into the sizes mapped out in the infographic above.
So whether an engineer is working on European machinery, they can rely on interchangeable fits thanks to the standardized sizes provided by ISO 773 and DIN 6885. No more guesswork about dimensions, just look up the diameter of your shaft and get the key width, height and cutting depth needed. Remove the guesswork from what is typically the most critical mechanical connection in a drive train.
How to Choose the Right Parallel Key
First you need a little background knowledge on what makes something a good fit for the application. How well does the key fit into the groove? Is it loose or do you have to force it? Tolerance classes, like the ones shown in the chart above, include a normal fit (N9) or a tight press fit (P9). Essentially this determines how easy it goes on or how much force it takes to put it on.
A loose fit might be nice during installation but it can cause microscopic movement which leads to fretting corrosion and eventual failure. That’s where a lot of folks miss it. You want the parts to fit, but you don’t consider how they will behave over time under changes in torque and vibration. The key itself has an h9 tolerance class. This means it should be the exact width of the keyway so there is no play to cause extra wear to the hub bore and shaft.
The cross section is not the only thing that matters; the end shape of the key also makes a difference. If your keyway was cut using an end mill, then the ends of the key would be rounded as well (Form A). An end mill is considered standard shop equipment, so this is by far the most common configuration. Using a side milling cutter creates a through slot, so your keyways will have square ends (Form B). These are often seen on large shafts; it’s easy to slide a key in from the side versus trying to drive it into a blind hole.
Last is the hybrid configuration: one square end and one rounded end. This lets you precisely position lengthwise while providing a hard stop against key migration under load (Form C). Get the wrong form wrong and you’re going to spend hours machining a shaft that should of been right the first time.
Beyond just size, there’s also a level of material consideration here. For most common industrial use case, standard C45 carbon steel provides adequate rigidity (it’s relatively easy to machine) as well as sufficient strength. But for high torque/impact load situations such as steel mills, you need to step up to something like 42CrMo4 alloy steel which has a higher hardness and will not deform enough to ruin the fit. Stainless steel keys retain integrity even under corrosion conditions, even if there is a slight reduction in shear strength different than their carbon counterparts. And in explosive atmospheres, you’ll often see brass or bronze being used for added safety rather then maximum shear power transmission capability.
One mistake made by amateurs is length. If the key is too short it can shear off when under maximum load. Since the keyway and key only make contact in a small area, they cannot take as much torque as you might think. One and a half to twice the shaft’s diameter is the general rule of thumb for length. Longer than the hub width doesn’t do anything except add more places for stress. Ideally you want sufficient surface area to spread out the force but not so much that you cut away at the shaft’s own structure.
A unique category includes feather keys, which are designed to transmit rotation but also permit some sliding on the shaft. For them, tolerances in the JS9 need to be just right so they don’t bind but have enough free motion. Too much freesliding results in precision lost in adjustable couplings; too little, it locks up a sliding mechanism.
That’s precisely what the standards avoid; a shared language among manufacturers around the world. The bottom line is when a properly designed keyed connection is installed it’s unnoticeable. You just use it to move power along seamlessly until the day comes to take it apart. If you select the proper form and respect tolerances then you will have reliable equipment without expensive down time due to avoidable mechanical issues. The humble piece of steel becomes an essential part of machine motion chain.
