Tapered keyway

How do you cut a tapered key slot in the hub for a gib head key ?

I have access to broaches to cut parrallel slots.

My first thought was to cut a parrallel slot and then use a tapered shim to angle the broach. Would this work ?

Paul.

Make a new bush with a slight taper to teh bottom of the slot.

If you are not using the very small broaches that don't have shims then I suppose a tapered shim to replace one of the supplied ones will do

How do you cut a tapered key slot in the hub for a gib head key ?

Paul,

You don't.

The keyway in the shaft and in the hub must be of even depth. When driven home the tapered gib headed key distorts slightly within the elactic limit of the material and locks the assembly tight.

Keys are made from a special grade of steel (not mild steel) in order to give them the abilty to be slightly distorted and not become loose over time due to creap.

Gib headed keeys are often difficult to remove as they grip so tightly. It is very easy to distort or even snap off the gib head as restorers of old engines often find out.

I can remember hubs with tapered keyways, only in bigger sizes tho', not in model sizes. I'm talking about 1/2" sq. or thereabouts. I'm afraid I can't remember how the keyways were made but have an idea that a slotter was used. I can well remember fitting the keys, which involved a file ,marking blue, and lots of patience.

Thanks for the advice.

(Reference: The key slot is for the flywheel on my 6" scale traction engine which is 1.1/8" ID x 3" long)

I have also found a copy of the Machinerys Handbook which has dimensions of taper keys (BS 4235) showing that the key and slot has a 1 in 100 taper but does not show how to make it.

Also found this site for fitting a gib key:

http://www.utterpower.com/fitting_gib_keys.htm

Edited By Paul Lousick on 11/02/2013 12:37:46

Paul ,

I've sent you a PM.

Richard.

Hi, well in all my years of being a maintenance fitter and fitting countless numbers of fans, motor pulleys, conveyor drum rollers and many other things using tapered gib head keys, I've never known them to be fitted into a boss that has not got a tapered keyway, and as far as driving them home so they distort is beyond me. They should hold in much the same manner as a Morse taper.

The two photos below show the keyway of a straight bore motor pulley, which was held on with a tapered gib head key.

The top photo is the front end where the key is fitted into and the bottom one is the back end and the photo below shows the key which held it on. While comparing the two ruler measurements showing about half a mill difference, the actual difference to the best of my measuring is 0.7493 mm over the bore length of 82.09 mm, which pretty much equates to the standard taper of 1 in 100.

The key fitted tightly at both the front and back of the keyway.

As far as I'm aware there is nothing special about the steel use for the key apart from the quality, and they will file as easy as mild steel.

When fitted, gib head keys should always be in contact with both the straight and tapered sides with the relevant keyways in the boss and shaft along the whole length of the bore of the boss.

Tapered gib head keys can be a problem to remove, depending on the conditions in the environment that the machinery is used and also how well (or badly) they are fitted. The one in the photo put up a bit of resistance but it had been outside in all weathers for well over 20 years. Soaking it with penitrating oil for a day or two and using a proper gib head key extraction wedge did the trick.

Regards Nick.

Nick,

I stand corrected.

I've always assumed that the taper has two uses. One -the key stays in the hub due to the slow taper holding it. Two- the hub is pulled tightly to the shaft ,this increases the friction and t/fore the torque transmitted, better than just shearing a square key, in a similar manner to using a grub screw. I've also seen grub screws onto the tapered key thru' the hub, not so bad if they can be seen.

Well described Nick, but Key Steel is a bit special.

There was a a thread on here last July, "Properties of Key Steel" which gives an account of the steel grade that is designated as Key Steel. It's medium carbon En 8, for the added strength but still easily worked so that you can properly fit it to the hub and shaft.

I recall the slotter operator clamping down the hub onto two "tapered parallels" to set the hub at the appropriate angle to match the taper in the hub keyslot.

Regards Bob D