round keys and keyways...will it work?

just curious if there is any good reason keyways have to be square, other than thats what standard stuff uses. i have a few times in the past had stuff i made where since i dont have a keyway broach and big arbor press i simply plunged and end mill half way into the bore, so it cut a half round internal keyway, then i used a ball endmill to cut the keyway in the shaft. i never realy did this on anything heavy duty, once it was a go cart steering shaft that was 3/4 dia and i used a 1/4 dia dowel pin for the key, seemed to work great, and there has been a few other times i used the same thing on half inch or so shafts with smaller pins for keys. well now i have a part that has a 1.5 inch dia shaft and bore i need join together, i was thinkin of using a half inch dia dowel pin for a key? it will be transmitting a good deal of hp, up to about 18 or 20 in the future possibly, at about 750 rpm non reversing and no shock loads realy, its just driving the bandwheel on a bandsaw mill. anyone have any input on this idea? seems like a good easy way to me but havent realy seen it used on anything other than my few small mechanisms. why dont manufacturers ever use it?

I think it would 'work', but it's not popular because the rounded edge is much more prone to shearing than a square edge. It 'invites' the hub to ride over the key. The actual portion of the round key that is doing 'work' is very small - just the narrow bit at the parting line. The further you get from the parting line, the less and less work the key does. Also, a square shape is much more forgiving than a cylindrical shape in terms of fitment - you will have to have a pretty precise matching of both rounded keyways. Otherwise you will very quickly get too much slop in the fit-up. If I were going to do this, I would try to use a normal end mill and orient the shaft and hub vertically in the mill so they could be machined simultaneously. This would give the best fit between the two halves.

I'm not sure why keys are used.....there are plenty of keyless shafts out there transmitting gobs of HP using just a shrink fit of about .001 per inch of shaft diameter. Isn't it such that they is not rreally intended to transmit power anyway?

I've often wondered the same thing. I've seen this arrangement a couple times in small, precision electromechanical devices, but never anything very big. For some reason, I'm thinking it's called a dutch pin, but I may be thinking of something else. A google search for "dutch pin" didn't return much that wasn't Von Dutch pinstriping or news of the Dutch football team pinning their hopes on something, so I could be wrong...

Andrew

dutch pin sounds sorta familuar, ya may be right there. i was figuring on cutting both my keyways a bit undersize then running a reamer thru them while they are assembled so a dowel pin would be a tight fit. i sorta see what d thomas means about not as much contact but it seems too that square keys often try and roll when under tourque so i figured that a round key with a tight fit wouldnt be any worst.

GregSY explained some of the drawbacks of using round keys. One technique that is fairly simple and avoids the disadvantages of round keys is to use a taper pin (assumes that one has a taper pin reamer - standard taper is 1/4 inch per foot).

Ream so that the pin is sitting proud of the joint so that there is something to grab when/if disassembly becomes necessary.

For what it's worth,the term may be "Dutchman". An old tool & die guy showed me how to make a fly cutter that consisted of a rectangular bar threaded onto a round bar, making a "T" shape. A hole was drilled to allow a screw to be threaded in on the part- line so the two would be locked and could not unscrew. The neat part of the process had to do with surface grinding the "face" of the bar, then clamping it to a faceplate and lightly turning the round section so it would truely be at right angles to the face. It did in fact work very well.
hms50

i have seen the method of using a set screw threaded in along the line as you mention, it was on alot of volvo hydraulic cylinders i worked on, where the piston threaded on the rod they locked it that way

Mark,
I don't see any reason why your method wouldn't work for the application you describe. Using a hardened dowel pin will help to prevent the hub/shaft from spinning in case of an unforeseen shock load.
I believe keys are made square because it allows for more mass in each of the mating parts, for greater driving force.

Greg,
Although shrink fits are actually stronger for driving loads, keyways are much easier to assemble/disassemble when repairs/replacements are needed.

Andrew and hms50,
The term is 'Dutchman' (don't ask me why!). I've seen these used to mount a hub or sprocket at the end of a shaft where very little driving force was needed. Typically, they are 3 or more blind holes, equally spaced on the intersecting line of the shaft/hub, drilled and tapped, and fitted with set screws.

RAS

heres how i picture the forces going thru a key, i made a fast little sketch the red line being what i imagine the force would be pushing as



and then here is the same but with a square key



these are just my sorta guesses from seeing how a worn key rolls in its keyway after a while and shows the pressure points

I haven't used round keys but did read a discussion on the technique. The author, sorry, I can't remember his name, seemed to think that a properly fitted round key was just as resistant to shearing as a square key of the same dimensions. He also stated that a round keyway introduced less stress in the wheel and bore than a square keyway due to the lack of corners which amplify the stress.

IMHO, I think the primary advantage of square keys is that they can be cut in both shafts and wheels more easily than a round keyway can. A standard end mill will do a square keyway at any location on a shaft, but a ball end mill is required for a round one. A shaper with a square tool will do a keyway in a bore and the depth is not critical but would require a precise form tool to do a round keyway and the depth of the cut is now very critical. The only place where a round keyway is easy to make is with a wheel on the exact end of a shaft so a drill can be used to create the keyway.

Paul A.

I've seen a number of them.....and they were all in places where you could just drill on the junction of shaft and wheel, then pound in the key.

Much easier than a square key, and no need for broaches etc. You can do the alinement and then drill, as opposed to carefully marking out and machining so as to keep the alinement.

I have also seen them as keys in tooling, used to aline a bushing in which a feature is put. Same deal, put in bushing and drill keyway on the junction.

i think i will go with the round key, i was gona stop by my old trade school and see if they could broach me a keyway or cut it in the vertical shaper we had, but workin fulltime makes it hard to find time to get over the. i think that they would have just as much shear strength since the part that would actualy shear is the same cross section, or actualy more since a round key is usualy larger than a square key for the same aplication, and the point about no corners for stress risers is interesting too

I have a 14" Elliot shaper that uses a round key and keyway to hold on the handle for the topslide-downfeed. They are not fun to remove.

what makes them so hard to remove?

I have repaired several High Horsepower (150+hp) drive lines with round keys.

A local dairy had several sileage choppers that had a taper splined U-joint yoke. You guessed it, they ran these joints loose and wore the splines off. The yokes were high $, the splined shafts were long, and these machines were old. The quick fix I applied was to drill 3 to 5 1/4" dia holes parallel to the taper and drive in dowels into the holes tighten the nut and send them back to the field. One came back several years later and the dowels were worn badly, along with shaft, enlarged holes installed bigger dowels and sent them off again.

Thankfully they finely scraped those machines. That sileage is very acidic and works on all the metal.

Ray

In turbine work, a dowel pin which is located so it splits a joint line is called a radial dowel. It is common in turbine work (hydro and steam turbines), but not used as a key, just as a locator.Typically, these are used between turbine casing feet and sole plates, and on some shaft coupling flanges.

I heard things referred to as "dutchman" over the years by my old man and other mechanics. Typically, a "dutchman" was something to correct a foulup or unforseen bust in dimensions. My dad used to look at an extra heavy saddle under a door and tell me: "Looks like some guy wasn't much of a carpenter- hadda put in a dutchman". Around machine work and powerplant work, we took a dutchman to mean an extra heavy shim or spacer piece that wasn;t in the original design- something like a machined spacer to take up a gap between a pipe flange and the flange on a pump casing; a Blanchard-ground piece of plate steel drilled for the mounting bolts and stacked in on top of a sole plate with the thin shims on top....


The type of "round key" is occasionally used in places where it is impractical to take things apart to put in a key. My own experience was with a large side-crank stationary steam engine in a remote location. The crank hub was cast semi-steel and the crankshaft was steel. The shaft was 12" diameter and had been pressed into the crank hub with the hub heated. There was a key about 2 1/2" square. I was re-erecting the engine and saw there was a gap of better than 1/8" down one side of the key/keyway and the key had actually deformed a bit off its end. There had been relative movement between the crankshaft and the crank hub. Probable causes: Engine took a slug of water thru the cylinder while running, or, generator paralleled into the grid out of phase. The hub probably took a 1200 ton locomotive wheel press to put onto the shaft, even with some heat on the hub, so the kind of shock load to cause realtive movement and opening of the key/keyway fit had to be huge. The fit between the hub and shaft was still good, but I did not think it was wise to rely on the interference fit to transmit the torque from crank hub into the shaft. I was concerned tha tonce the fit had been broken by shock load, it's ability to hold things together was compromised. Either way, I had to secure the crank hub to the crankshaft against further relative movement. I opted to put in two round keys. To do this, I set up a big mag base drill (required a fixture plate and a few smaller tapped holes into the crank hub's "web"). I then drilled progressivley larger holes until I had drilled two holes a 1 7/64" diameter x 6" deep- half on the hub, half ont he crankshaft. These were then reamed and cleaned up with a brake cylinder hone. Two dowels were turned from some 4140 (or whatever truck tranny shafts happen to be). Final fit was made by polishing the dowels. We then had the artificial inseminator show up with his liquid nitrogen to chill the dowels, and we warmed the hub/shaft with a rosebud (oxyacetylene torch with heating tip). We heated the crank hub/shaft areas until they were warm to the point you could barely keep your hand ont hem. The dowels got chilled down in the liquid nitrogen, dunked in alcohol and then slid into their holes. We finished erecting the engine and I got the valves set based on the "new" dead centers since the crank hub had been moved relative to the shaft. The engine ran fine and pulled load, and I dumped it off line wiht full load as part of startup testing. That was years ago, went home and never heard anything further.

Would I recommend the round key in place of a square key ? For very light loads/small shafting, I would use a round key. For anything seeing cyclical loads or anything beyond very low fractional HP, If I COULD put in a square key, I would prefer to. Will the round key transmit the same torque as a square key of same width (diameter) ? In theory, yes. The shear area will be the same. Reality: The round key relies on a very good fit between the bore int he hub and the shaft to avoid a "rolling up" or wedging action. To avoid any chance of the round key rolling up or wedging, the fit between the hub bore & shaft has to be very close- a drive fit or light interference fit being optimal. For everyday use, where someone takes a pre-bored pulley or sprocket to fit on commercially sized shafting, the fit may be a little on the loose side as the pulley or sprocket hub may slide on with little or no force needed. This would allow some relative movement between the hub and shaft. Over time, under changing loads, the round key will start "working" or wedging/rolling. It is probably along the lines of the guys who wear a belt and suspenders, but if I had to use a round key on a pulley or sprocket with commercial shafting, I would lock int he key. To do this, I would drill/tap a hole half on the end of the round key, half on the hub or shaft. I would tap the hole as deep as I could, so I could use a long setscrew and a short locking setscrew. This would lock the round key against any rolling or "chucking".

There is one major manufacturer who built their
machines this way for many years. Nearly
every person on this board has used one of them,
and probably about a third of the folks have
actually seen one of these round keys used in
practice.

Which manufacturer, and where are they used?
Hint, two round keys per machine, and only two.

Jim

forget the keys. RINGFEDERS rule Martin

What you are describing is called a "scotch key", do not know why it is called that. Check out this link, scroll down to # 49 and there is a reference to it.

http://www.ploughbooksales.com.au/003472.htm

The best way to make sure that they are tight, is to drill the hole when everything is assembled and then either pin it with a hardened roller from a bearing, or drill and tap it and then put in a "grub" screw. Either way will take a hell of a lot of pounding/shock before they come loose. When or if,they do loosen up, drill and tap next size up and just keep motoring along.
(That book is worth every penny you pay for it, it's just about the bible - not quite, the Machinery's Handbook is the actual bible - this one runs a close second.)

regards radish

Jim said;

"There is one major manufacturer who built their
machines this way for many years. Nearly
every person on this board has used one of them,
and probably about a third of the folks have
actually seen one of these round keys used in
practice.

Which manufacturer, and where are they used?
Hint, two round keys per machine, and only two."

Methinks my SB9 has two and only two round keys, one on the cross slide crank and one on the compound crank.

Paul A.