Flywheel keyways

[Maurice Cox 1Participant @mauricecox1]

Engineering reference books give the taper for flywheel keys as 1 in 12. However, no mention is made of making the keyway, in the hub of the wheel, match this taper. Every model that I have seen described has the keyway parallel. However, I have two books with sectional drawings of the levers on Corliss valve spindles. They are keyed to the spindles, and the keyway in the levers is plainly shown as matching the taper of the key. If this was done on these levers, then why not on flywheels? I have seen setscrews fitted to hold keys in small wheels.There would be less chance of the keys coming out if they were driven into a matching taper. Are the keyways in full size practice ever cut to match the key?

Regards Maurice

[Maurice Cox 1Participant @mauricecox1]

[Anonymous]

Following a recent discussion on the TractionTalk forum (see the last page) it seems that the flywheel keyway should be tapered to match the key. You learn something every day!

**LINK**

I suspect that in the model world parallel keyways are used as they are easier to machine, and long term reliability is not an issue.

Andrew

[Keith LongParticipant @keithlong89920]

From my reference books in the real world (ie not model) the keyways in both shaft and hub are parallel as well. The taper on the keys is given as 1 in 100 though not 1 in 12. The old "UK" standard taper was 1 in 96.

I think your 1 in 12 figure is the taper on the end of the shaft, the reference says that the keyway in such a shaft is "parallel to the side of the cone" and as the "depth of the keyway at the sides therefore alters with the change of section. For determining the correct size of key, the nominal section of the shaft is taken to be that at the larger end of the cone". (Kempe's Engineers Year Book, 1965)

[Paul LousickParticipant @paullousick59116]

Machinery's handbook shows a tapered keyway in the hub.

Easily done by slotting at an angle or using a standard keyway broach by making a guide bush that has a tapered slot.

There is a lot of force on a flywheel key and if only secured by wedging against the corner of a straight keyway in the hub it may come loose. My Ruston TE has 2 x tapered keys at 90 degrees to each other. Also adding a cap on the end of the crankshaft to retain the flywheel in case the keys come loose.

Paul

[JasonBModerator @jasonb]

The 1 in 12 is actually 1/8" in 12" = 1:96 these are imperial key tapers, metric use 1:100

Funny you should asj but I have just machined two flywheels and a pully that will need tapered keyways to suit the supplied tapered gibb head keys. I will just make a bush with a 1:96 tapered slot to guide the broach as I have done in the past

If you need to make the tapered keys then an easy way to set the taper is like this, just space the packer the correct distance along the rule. For a 1:96 taper I would place a 1/16" packer 6" from the unpacked support. The rule acts as aparallel to support your key blank. Some may say poor mans sine table but what else will fit when machining a narrow key

 

 

J

 

Edited By JasonB on 03/02/2015 07:49:28

[Paul LousickParticipant @paullousick59116]

Hi Jason,

You are correct about the taper ratio. My Machinery's Handbook is the metric version.

Instead of raising one end of the bush with the 1/16" packer over a measured distance (sine gauge method) you can use the tapered key as an angle gauge. You do not have to measure anything. It is already at the exact angle that you are trying to achieve.

Paul.

Edited By Paul Lousick on 03/02/2015 08:49:36

[Chris GunnParticipant @chrisgunn36534]

I used to work on nailing machines with an action like a press brake, the mechanical clutch hub was fitted to the 2.5" up to 3" shafts with an 8" long taper key, the 7" long clutch hub having a matching taper. It could take half a day to properly fit the key to the hub and get the hub in the right position along the shaft. The whole assembly transmitted 12- 30 HP depending on the machine size. Sometimes we needed to change the hub, and our 50 ton press would not always be enough to break the joint. Our local power transmission engineers broached the taper keyway for us. Anyone wanting a taper keyway in a flywheel should check out their local power transmission company, they may do it on the premises, and if not will know a man who can. With the right tackle this is a 5 minute job, so should not be expensive. As already mentioned, It is inevitable that a flywheel doing any work will come loose if secured with a taper key in a parallel top keyway, as there will only be point contact between the top of the key and keyway, and in time the key will loosen and the flywheel will slide along the shaft.

Chris Gunn

[JasonBModerator @jasonb]

Quite possible Paul and easy enough on larger keys but the one I was cutting in that photo was only 1/16" wide x 5/8" long so not easy to hold and run a DTI along. By mounting the vice lengthways along the mill table the bush can be held top and bottom against the jaws and a 3/32" packer set below the vice 9" from one end I can get a 1:96 taper.

And to be sure of a good fit blue the key and fit it to the flywheel, if making your own key then leave it on teh end of some rectangular section key steel so you have something easy to hold and pull it out with while fitting

[Paul LousickParticipant @paullousick59116]

I like the idea of leaving extra steel on the end of the key while machining to the correct fit. then cut off after it is correct.. Paul.

[Maurice Cox 1Participant @mauricecox1]

Thank you very much for all the feedback about this. I have wondered about it for some time. Sorry about the 1 in 12 taper mistake. I should have checked before I started. I have just checked with my 1897 edition of Henry Adams Handbook for Mechanical Engineers. It says:-

" Ordinary Proportions of Keys.

Width of key 1/4 the diam. of shaft up to 4 inches

1/5 the diam. of shaft 4 inches to 8 inches

1/6 the diam. of shaft 8 inches to 12 inches

Key square at thick end. Taper 1/4 inch per foot. One third of the thickness let in shaft, remainder in the wheel."

No mention of whether to taper the keyway in the bore though, hence my enquiry.

Thanks again Regards

Maurice

[Max ToleranceParticipant @maxtolerance69251]

The reason for fitting taper keys to flywheels or any other highly stressed part was because the fit had to be well nigh perfect. A parallel key should be fitted leaving a slight gap on the top and it should only bear on the side of the keyway. This is fine for small pulleys etc where the shear stresses are relatively low. However if you have a 30 tonne or more flywheel whizzing round at 60 RPM the stresses in the hub are huge therefore a taper key was fitted and would be fully scraped so as to touch on all four sides all over. This is only possible using a taper. As an apprentice (and after) I regularly used to fit taper keys to paper machine cylinders, brakes etc. we also fitted them to line shafting pulleys. Not many people had the patience or ability to fit them correctly so I used to be selected almost every time. The trust that the foreman had in me was such that in my last year as an apprentice I was asked to fit the keys into the brakes on the fifty tonne overhead cranes. I spent a week up there with a file, a tin of marking blue, a slide hammer and my trusty scraper. Afterwards the crane was put to some pretty severe testing involving large concrete blocks and sudden application of the brakes after fast down. The whole building shook, the supports bounced and banged, the cables groaned and the brakes got rather warm……………… but the keys stayed where they were.

Obviously in a model there is only a requirement to copy full size and the stresses do not require quite such a good fit. But that was the reason for the different keys in days gone by.

[Nigel McBurney 1Participant @nigelmcburney1]

On some hot bulb engines of the 1890s,the flywheel was secured with one fitting tapered key and keyway,and another tapered keyway in the flywheel at 90degrees to the first keyway,with a tapered key,instead of a mating key way in the shaft a flat was machined on the shaft,the second key was driven in tight,I believe this method was used,as some of the engine makers machinery was not very good at the time,and they would have found it very difficult if not impossible to machine two pairs of mating keyways,the second key was probably required as single cylinder engine only fires on one stroke in four,compared to a steam engines two beats per rev.

[mechman48Participant @mechman48]

Ditto Max

Part of my apprenticeship was spent fitting taper keys to shafts for brake presses, o/head cranes etc & as stated fitted to all sides… removal was with a slide hammer.. or banana drift.. but once they were in they stayed in so you had to get positioning right before hammering home..

George

[herbert punterParticipant @herbertpunter99795]

I the mid 1970s I repaired a 1945 vintage single cylinder Lister generator. The big end was worn and the rings were letting by. I got the crank reground and the cylinder rebored, the parts were available off the shelf. The key was a bit loose so I got a new one made. The flywheel was about five foot in diameter on a long shaft that could be fitted with a large handle so one could get it going if the batteries were flat. The chap who had it told me that the key would now and then fly out whilst running. He would know when it had happened because the note of the exhaust would change slightly although it kept running in spite of the free wheeling flywheel!

I offered to have a go at it but he refused saying it didn't happen often and what was the harm? I pointed out the potential hazards but he said the flywheel showed no inclination to 'walk' along the shaft. I looked at it and had to agree that it seemed to be staying put so we left it at that.

I'm not making any point, just giving my experience for interest.

Bert

[Gordon WParticipant @gordonw]

I'll add my little bit- I also fitted taper keys as an apprentice. I don't see any mention of Gib keys, ie. one with a head. Assume they were gibs if removed with a slide hammer, we did not have any such luxury! Just mentioning gib keys as some people may not realise they exist, very difficult to make in model sizes. If the key is for driving ,or stopping, then they must fit on the sides, top fit is secondary, and is as much to stop the key falling out as anything else. And Woodruff keys are for location, not for driving. Except in very small sizes. Just thought I'd get that in again.

[mechman48Participant @mechman48]

Agree with you Gordon.. they were gib keys, otherwise you wouldn't be able to use a banana drift, if the tapered key was fitted to a flywheel that was to be flush with the shaft end then it was drilled & tapped for sliding hammer fitting & removal as using a banana drift would bend the unsupported key, the gib would then quite often be sawn off ( Elfin safety.. entanglement ! ). Since those days taper lock bushes are the norm for fitting flywheels, couplings, pulleys etc.

George.

Edited By mechman48 on 04/02/2015 11:19:31

Edited By mechman48 on 04/02/2015 11:20:30

[Ian S CParticipant @iansc]

Not a good photo of the key, but this is the one I made to fit the flywheel on a Stuart Turner S9 that I rebuilt.

Ian S C

[mechman48Participant @mechman48]

Ian

No disrespect intended & maybe my view of the pic; but it looks like the key is upside down in the keyway, with the gib facing the shaft..?

I have always fitted gib keys with the taper of the key matching the taper in the flywheel, with the flat bottom of the key fitted into the parallel keyway on shaft.. if it's my eyes then I stand suitably admonished.. may be a pic from an oblique angle to show the key better would clarify, as I see it now it immediately hits me as something wrong?

Respect..

George.

[Ian S CParticipant @iansc]

George, no it's round the right way, just the angle of the photo. Thought I had a better one than that. The motor is in a local museum now(it was the museum owner that got me to do the rebuild).

Ian S C

[mechman48Participant @mechman48]

Ian

Thanks, it confirms that my eyes plus the angle of the pic are what confused me; I guessed that a man of your calibre would have it orientated correctly.

George

[Ian S CParticipant @iansc]

It's the first one that I'v made that small, the smallest one before that was 1/2" wide key way. My latest hot air engine has my first attempt at a taper lock fitting, with any luck gone are grub screws, or keys.

Ian S C

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