Shaft coupling, keys or keyways could be root of vibration

Jul 1, 2016 by Rich Merhige

When it comes to vibration, the devil’s in the details. While it’s pretty easy to pinpoint misalignment, propeller issues or a bent shaft, less conspicuous components like shaft couplings, keys and keyways can also cause issues.
In mechanical engineering, keys connect rotating elements, in this discussion, couplings to shafts. The key sits in a keyway, and acts as a mediator to prevent relative rotation. While published standards only address key and keyway dimensions, key material, as well as their shape and installation, are equally important.
Keys can fail if they are overloaded. To avoid this, the correct material needs to be selected for the key. To properly transmit torque, and provide adequate support, the key material should be as strong and hard as the shaft material. Since part of the torque is transmitted via friction between the shaft and keyway, according to published key stress calculations, the key material needs to be as strong as the shaft.
As mentioned earlier, key geometry and fit are important specs as well. With proper shape and fit, the hub is prevented from rotating on the shaft, which could cause hammering on the key. This would eventually cause clearance to develop which would eventually lead to shearing of the key, or allow for corrosive water or gases to penetrate. A proper fit would have the following characteristics:
l Tight in the shaft keyway, with a sliding fit, without clearance present.
l A slight amount of clearance will be present between the top of the key, and the bottom of the hub keyway.
l The key should extend inward from the shaft end and past the hub end.
l Keys should have chamfered corners and not be squared. Sharp corners can cause stress on the shaft which could cause it to fail while transmitting torque. SAE standards specify rounded forward edges. It could also be beneficial to hollow out the keyway’s forward end.
If a key has a round end, it should extend beyond the hub by at least the round end. This will allow torque to be transmitted over the entire length of the hub. If not, small alternating motions of movement can impact the shaft, leading to fatigue failure. Having the incorrect key length can also cause imbalance, which would present itself as vibration at high speeds.
When propellers are lapped onto shafts, a gritty paste is used on the two surfaces to get a good fit with at least 80 percent contact to prevent the propeller from being off balance. The only problem is, keyways can become compromised because material is removed during the lapping process. Keyways should always be rechecked to see if any machining is necessary to perfect the fit.
In regards to propeller shaft couplings, there are several different kinds, both keyed and keyless including: tapered bore, split straight bore, and straight bore, among others. While the straight bore couplings are the easiest to machine, they can be the hardest to fit shafts to, and, very difficult to remove if not split. Tapered couplings actually guide the shaft into place during fitting, and, come apart very easily. Keyed, tapered bore couplings are most often recommended by industry experts for propeller shafting. Besides allowing for easy fitting and disassembly, the nuts on the shaft ends prevent the shafts from becoming displaced when the vessel is in reverse. Retaining pins, if used properly, provide this same function on straight bore couplings.
With components as seemingly simple as keys and keyways, it’s crucial to recognize that every part of a mechanical system plays a very important and individualistic role that’s part of a larger working system. Even if something is slightly off, the system can be affected, and symptoms will manifest as vibration, premature wear of parts, and, in worse cases, failures. Staying on top of everything can seem overwhelming and daunting, so the best way to combat that is to learn about the system at hand so you can make educated decisions about maintenance.

Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale. Contact him through