View Full Version : Grrrr - Woodruff keys

John Stevenson
03-14-2005, 06:46 PM
Why do manufacturers insist of fitting woodruff keys to shafts ??
They have got to be the worst engineering cock up of the 20th century.
You have got a circular cutter that plunges in to a greater depth than needed to present a decent length of key on the surface and in the process it leaves nice sharp corners right down into the shaft to act as stress points.

We won't mention problems in fitting where the key rolls out the slot and either falls down the back of something either immovable or expensive, or just falls out and it's not missed.


Here's a picture of what happens to these stress points at the root of a keyway.
Shaft is the armature out of a 7Kw DC fork truck motor.
A new motor is about $1200 if available.


Partial repair, shaft has been turned down to mach the taper about 30 thou undersize.
This is so the weld transition point is below the new surface so you don't get any scabbing or flaking.


Turned back down and polished to fit the gear.
Keyway replaced with normal feather key.
This gives a longer contact area that the original woodruff key and minimises stress points as it's not so deep.

BTW this isn't a rare example, these DC motors come in all the while in this state.

John S.

Allan Waterfall
03-14-2005, 06:56 PM
Nice repair.

When you find that floor you'll find no end of woodruff keys. http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif


03-14-2005, 06:56 PM
John, is the woodruff key just an expediant production method. They do tend to screw up...maybe inbuilt obsolescents.
Still, the work must help to pay the bills http://bbs.homeshopmachinist.net//biggrin.gif: http://bbs.homeshopmachinist.net//smile.gif
I do know that they can be a bi*ch to fit grrrrr.

cheers, Ken

Alistair Hosie
03-14-2005, 07:10 PM
Fantastic job John! well done. You just saved someone a few bob there all right big thumbs up.regards Alistair

03-14-2005, 07:13 PM
Nice repair job! I've got a similar gripe about pipe theads. Some whacko marvels at the efficiency of the cork and comes up with "The threaded cork". The bonus is it comes with free a spiral leak path that needs to be sealed.

03-14-2005, 07:16 PM
Nice job John! I may have a couple of fixes to do later this summer. (Model T axles) When I was giving it some thought, I wasn't sure if I should cut the key way on the same angle as the taper. You answered that question. As I think about it more, there's no other way. http://bbs.homeshopmachinist.net//biggrin.gif

Thanks for the great pictures.

Spin Doctor
03-14-2005, 07:32 PM
Well let's think about the causes of this shaft failure. Number one I suspect that the fork truck operator was doing what most truck drivers do. Trying to make time. After all why come to complete stop when you can just throw the truck into reverse or forward when moving in the opposite direction. Do this three or four thousand times (or more) and its not surprising that the shaft failed at the natural stress risers. And if a straight key is subjected to the same abuse then it will also fail usually by washing out the key way. For every Woodruff key I have seen fail either by shearing or shaft failure (mostly shearing) I have probably seen 10 straight keys that have failed. Also this shaft had a taper on the portion with the keyway. One wonders if the gear/sheave was loose and rattling on the shaft. If the component was tight on the taper it should of been acting to constrain the shaft and help prevent such a failure. But one thing I really like about Woodruff cutters is the use of them to cut straight keyways. They produce a more accurate width keyway with smoother sides and the sharp corners do not go all the way around the keyway slt. Plus if one is creative they can also be used to cut parallel key splines as long as they are of the side or top fitting variety.

John Stevenson
03-14-2005, 07:58 PM
It depends on whether the keyway in the internal part follows the taper.
if so then the male part has to follow but I have seen many parts where they have broached straight down to producr a non tapered keyway.
In this case you have to cut the keywat paralel to the shaft axis to match.

Spin, I agree with you that the taper should have taken most of the load, some of these don't have a keyway, just the taper.
Mind you I have seen my fair share of these just twisted off as well.

We are seing a lot of new mtors comming in that have keys that have been stamped or pressed. Not chaep and nasty stuff but well formed.
They are doing this so they can put a radius on all the edges and the slot is being milled with cutters also with a small radii on them.

I'll post a pic of one of these keys tomorrow so you can see an example.
I must admit we don't see many woodruff keys here now, they used to be very common.

John S.

03-14-2005, 11:04 PM
Yes, I just love the way the woodruff key rocks when assembling components onto the shaft. They test my patience ...often.
Poor technique?? http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif
I forgot to comment on your fine work John.
Good stuff!!
The locals must think ( know ) they are in heaven having access to a machinist of your calibre.

cheers, Ken

03-14-2005, 11:26 PM
Nice job!I too hate woodruff keys,most I see are on $$$$$ hydraulic motors.Just how do they expect a 5/16 key to take 800 ftlbs of torque?
Only other place I still see them is on engine crank snouts,most of the time pounded out or like the one in your pics.

03-15-2005, 03:57 AM
my goodness , you fixed all that and did not just re-key it @ 180degree ?

there is still some good shaft left,lol.

nice fix.

Norman Atkinson
03-15-2005, 01:25 PM
Nice one, John.

Obviously, there is a breed of engineers who haven't a clue about lapping tapers.
The Woodruff should only be an ornament.

My last one- end of Myford leadscrew- refuses to come out of hiding. The one on my Pultra 1750 has probably eloped with it.

A bit like the two seat Cessna which crashed on an Irish cemetary. 1758 bodies have been recovered so far and the Guarda are continuing the search at first light tomorrow.

One for St Patrick's Day!!!!!!

03-16-2005, 12:22 AM
I have always had good luck with small block chevy engines, so awhile back I bought a GM Performance Parts crate engine (HT 383). One hell of an engine, until the crank broke at the harmonic balancer keyway (woodruff key) at only 12,000 miles. You can see the fan shaped crack pattern that originates in the bottom of the keyway and extends about 320 or so degrees around to the point where it finally snapped off. I'll probably have to eat the cost of a new crank and associated parts to fix it as it had a 12 month(I've had it over two years) 12000 mile warranty(just about exactly the miles I have on it). $5000 invested in the engine and now a $440 tow bill to get it home(100 miles on a sunday night) This is killin' me fellas!

03-16-2005, 04:40 PM
The theory behind a woodruff key is that it will shear first thus saving the shaft and componet from damage. I don't like them either but many problems are cause by not using a new key everytime or dressing the slot before assembly.

03-16-2005, 09:36 PM
John...nice work!!!! Thanks for the tip about turning the shaft down some before welding. I've never heard of that before but it sure makes sense. Ha...That's why I come here all the time...learn something new nearly everyday!

03-17-2005, 11:15 PM
I don't get what the problem is. Woodruff keys are EASY money. I just keyed 100 shafts this week with four keys of two sizes. Total gravey. I made the keys to spec. so if they fail over time then they will have to just pay me to make more.

03-20-2005, 04:59 AM
John S

Out of interest, what welding process did you use on the shaft and what did you use to cut the weld when it was turned down??

In my limited playing around, the MIG welded filler material is way too hard to be turned by HSS or my low quality Chinese TCMT carbide tips on my 6" centre height equally low quality Chinese geared head lathe.

03-20-2005, 05:20 AM
I was always taught that any "taper shaft" key whatever kind it is, is only there to prevent the shaft from rotating whilst the nut is tightened up. The taper does "all" the driving.
I run a 1936 Austin 7 car and these tapered keyed joints are used in all sorts of critical places, such as the ends of the axle half shafts and the crankshaft to flywheel joint.
The only sure way of obtaining a good joint is to lap the two part together with fine grinding paste on final assembly.
And then when you think you have fully tightened the nut, go back and do it again just to make sure !
I cannot believe the number of people who still try and tell me the key does the driving, if that was the case, machine tools would have a key on the spindle taper wouldn't they.


Spin Doctor
03-20-2005, 06:48 AM
Oh, like an L Series Long Taper LAthe Spindle Nose

John Stevenson
03-20-2005, 06:49 AM
Just plain MIG general purpose wire.
Most motor shafts are plain cold rolled steel.
It's not until you get up to big sizes they start using better materials.
Tooling is just indexable tips.
I use a lot of the triangulat TPUN series as they are cheap and you get three bites of the cherry.

I also have a small 1/4Hp motor with a 4" diamond wheel on it that I use for just lapping old tips up.
It's not a question of being mean but when you get interuped cuts like a welded shaft it's very easy to chip or blunt a tip.
Reusing the old tips until you get under the skin can save you money.
On some big jobs I can loose 3 or 4 tips getting cleaned up.
That's better in my pocket than theirs http://bbs.homeshopmachinist.net//biggrin.gif

John S.

Charles Ping
03-20-2005, 08:12 AM
[QUOTE]Originally posted by pgp001:
I cannot believe the number of people who still try and tell me the key does the driving, if that was the case, machine tools would have a key on the spindle taper wouldn't they.

I'm with you here Phil.
Practice tends to show that the driving key on an R8 Bridgeport taper can be left out with little problem.


03-20-2005, 11:27 AM
Spin Doctor

Thanks for re-inforcing my point about the key on an L series taper, its only there to prevent the chuck from spinning whilst you tighten the large backnut which pulls it onto the taper.


03-20-2005, 11:36 AM
Good point on the L tapers,but it also keeps the spindle from being destroyed if the locking ring should work loose,I have had this happen before.

I too will lap in a taper,on the larger sizes and on machinery subject to shock loading I go one step further.I heat the female to 250*F,then assemble and torque the nut,they never come loose unless heated while under pressure from a press.