A keyway is a square channel machined into a round steel bar, usually a shaft of a rotating machine component. The channel, or keyway, gives a pulley or sprocket a spot to lock onto the round bar using a square piece of metal known as a key.

The purpose of the key is to make a solid connection in the assembly by snuggly fitting between the shaft and the gear. Numerous components, such as axles, crankshafts, and drive shafts, use keys and keyways to secure and transfer kinetic energy to other components during rotation.

In car engines, the harmonic balancer is held in place by a keyway that was placed into the crankshaft's snout end. This balancer powers the belts that drive all of the engine's accessories, including the alternator, air conditioning compressor, and oil pump.

It's important to note that these keyways are usually machined into the appropriate components prior to hardening; the size of the channel in the shaft and the size of the key must be appropriately matched; otherwise, the key would break, harming other components in its vicinity.

In this article, we'll discuss keyways; more specifically, we'll discuss crankshaft keyways, what they are, what their purpose is, and how they can be repaired in case of damage.

Crankshaft Keyways: What Is Their Purpose?

By definition, keyways are machined grooves in shafts, like a camshaft or a crankshaft, and various mating parts, like flywheels and gears. They're machined to be the same width as they're each supposed to receive a part of the key, holding it to another part.

The main purpose of said keyways is to prevent components from slipping on and rotating independently from the shaft they're mounted on, which adds torque capacity in driven systems.

Car engine parts are the perfect example of their application. A crankshaft that's connected to pistons via connecting rods has a timing gear, which is connected to a camshaft within a cylinder head via a timing chain or a timing belt. When the timing gear rotates, it drives the timing chain, which drives the camshaft that's in charge of controlling the valvetrain.

It's important to note that the camshaft, along with the timing chain, the tensioners, connecting gears, and the camshaft, are all secured to their mating components via keys and keyways. Without these keys, the timing gear on the crankshaft wouldn't rotate with the crankshaft.

Without keyways, we would have to rely on screws to maintain a rigid connection, and the holding power of the screws would determine the amount of torque the assembly could withstand. However, the torque capacity is equal to the shearing resistance of the key when the keyway is machined into the shaft and mated with the appropriate components with a key.

What Is the Difference Between a Key and Keyway?

We recognize three different fittings regarding crankshafts: a key, a keyseat, and a keyway. The latter two are used interchangeably by laymen, but there are some differences between the two, despite serving the same purpose.

A key is a rectangular piece of steel stock that interfaces with the keyseat and the keyway, creating a solid connection between the two and allowing torque transmission by preventing the relative rotation between two components.

The shaft and the rotating component — like a gear, a crank pulley, or a coupler — must both have a keyway and a keyseat for the key to work effectively. A crankshaft keyway is actually a keyseat groove or a pocket machined into the shaft. On the other hand, keyways are slots machined into a hub and provide space for the key to be inserted. The entire system is referred to as a keyed joint.

How to Repair a Keyway

Oversized keyways are a common issue when it comes to rotating equipment; the shaft-mounted keys can endure the shearing forces and strains in most cases, and when damage occurs, it usually affects the keyway, which may become oversized.

When that happens, it's necessary to turn off the machine, disassemble it, and remove the keyed joint — both the shaft and the hub. Machining a new keyway or a keyseat isn't an expensive or overly complicated task, but the entire repair process can be extremely time-consuming.

A quicker way to repair the keyway would imply the use of metallic polymers, which eliminate the need for extensive disassembly. In most cases, the damaged shaft needs to be exposed, with a key set in place using an adhesive, like JB-Weld, or Loctite. The polymer is then applied to the oversized keyway around the fixated key.

That way, when the polymers solidify, the new keyways will be a perfect match for the existing key, as there were no dimensional changes associated with machining a new crankshaft keyway or using a new key.

How Do You Put a Woodruff Key On a Crankshaft?

Woodruff keys are a fantastic alternative to parallel keys and keyways. Putting a woodruff key on a crankshaft implies cutting a half-moon slot into the shaft with a woodruff cutter tool. The crankshaft woodruff key's curved section would be pressed into the keyseat, while the key's flat side interfaces with the slot in the hub, gear, or bushing.

 

Make sure a woodruff key is firmly seated in the key seat to maintain it in place while attaching a matching component. Woodruff keys frequently need to be pressed into place because of the strict tolerances under which they are manufactured. To accomplish this, lightly tap the key's flat, projecting face with a hammer or mallet until it is firmly installed.

Summary

A crankshaft keyway is a square channel machined into a round rotating shaft, interlocking it with a mating component and preventing the two from independent rotation relative to one another. Parallel keys and woodruff keys, and respective keyways and keyseats are the most common types of fastening shafts with other elements. If you're interested in learning more, from Reid Supply you can view woodruff keys here.