Hi all

Not DIY at all really, but knowing the group's expertise in mechanical
engineering.......

If you have a keyway, why would you also want an interference or transition
fit on a pulley?
The drive suppliers we use are offering gearboxes with m6 tolerance output
shafts.
This gives a tolerance that has an upper limit larger than the nominal bore
size.
This implies to me that a pulley fitted to this shaft will tend to have at
least a possible interference on the shaft rather than being a free fit
(because the standard hole tolerances start at the nominal size and go
larger).

Am I missing something here - is this standard practice to maybe have to
lump hammer a pulley on?

TIA

Phil

TheScullster wrote:
Hi all

Not DIY at all really, but knowing the group's expertise in mechanical
engineering.......

If you have a keyway, why would you also want an interference or transition
fit on a pulley?
The drive suppliers we use are offering gearboxes with m6 tolerance output
shafts.
This gives a tolerance that has an upper limit larger than the nominal bore
size.
This implies to me that a pulley fitted to this shaft will tend to have at
least a possible interference on the shaft rather than being a free fit
(because the standard hole tolerances start at the nominal size and go
larger).

Am I missing something here - is this standard practice to maybe have to
lump hammer a pulley on?

It can be. If ANY wobble exists it can wear things badly.
The aim is to just about be able to assemble and disassemble, but
otherwise have the parts essentially locked together.

A key can shear under a jerk..

Normal practice would be to heat the pulley and drift it into place
carefully, or use a press.

TIA

Phil

On 3 Oct, 11:20, "TheScullster" wrote:

If you have a keyway, why would you also want an interference or transition
fit on a pulley?

Both. You'd want to choose the fit according to your needs, _then_
worry about whether there's a keyway to lock it against rotation.
Whether there's going to be a keyway or not doesn't change the fit
needed.

Typically you'd use a transitional fit. An interference fit would only
be needed if there's some reason to lock the pulley with better
precision, such as a need to avoid wobble with a high accuracy.

The drive suppliers we use are offering gearboxes with m6 tolerance output
shafts.
This gives a tolerance that has an upper limit larger than the nominal bore
size.

Tolerance isn't interference. Certainly don't just ask for a sloppy
tolerance if you really do need a reliable interference fit.

This implies to me that a pulley fitted to this shaft will tend to have at
least a possible interference on the shaft

_Possible_ interference.

You deal with this by first checking the comparable tolerance sizes
for the pulley (maybe it doesn't go that small - read your Machinery's
Handbook (and go get one if you haven't already - eBay's cheap
enough). You can even take a statistical approach and calculate the
likelihood of a non-fit actually happening [1].

If there is a risk of parts not fitting by hand pressure, then you
deal with it. You choose to either check fit before assembly (bin the
rare non-fitters), to play mix-and-match with paired parts (slow),
hammer them in (the British Way) or hand-ream when needed (even
slower). It's _essential_ to train your line operators to do the right
thing. If throwing it away is the right thing (for the overall
process), then don't force it to fit! You'll make a duff assembly that
may have a short lifetime, or might not be field serviceable.

The engineering way is to correctly spec the tolerances on things that
need to fit together. That's what this stuff is _for_, all those
standard tables of acceptable limits. Those tolerances are well worked
out - things do still fit together (i.e. tolerances are deliberately
asymmetrical, so holes are always bigger than shafts), so long as
everyone does what they ought to. (I don't have my MH to hand though,
so I can't quote figures)

[1] This used to be a good technique, but as manufacturing gets better
it doesn't actually work any more. Modern machining no longer gives
an imprecise size with a Gaussian distribution, it now gives highly
precise sizes that may be systematically inaccurate. If you had to
reject 1 in a 1000 and that's really how rare it was, then you could
live with it. These days though your SPC-using manufacturer probably
makes parts that are highly repeatable, but they might _all_ be wrong!

On 3 Oct, 11:29, The Natural Philosopher wrote:

A key can shear under a jerk..

sigh, no I can't be bothered.


Normal practice would be to heat the pulley and drift it into place
carefully, or use a press.

********. Maybe in your one-off backyard Alex Weyger world, but not in
production engineering.

Andy Dingley wrote:
On 3 Oct, 11:29, The Natural Philosopher wrote:

A key can shear under a jerk..

sigh, no I can't be bothered.


No? I have had one do it.

Force fit on a lawnmower disc. Got loose, hit a stone, and hey presto.
Disk spinning freely on crankshaft, and key sheared off. Better than a
new crankshaft I suppose.


Normal practice would be to heat the pulley and drift it into place
carefully, or use a press.

********. Maybe in your one-off backyard Alex Weyger world, but not in
production engineering.


Production engineering you would slam the thing on so tight on it would
never come apart.

It all depends on what its for.

Anyone who has had exposure to cars for long enough including such joys
as non force fit wire wheels on splined hubs, and other wheel bearings
and parts that ned all you can throw at them to torque up enough to
stand high performance use, will tell you that the tighter the fit the
better.

"Andy Dingley" wrote

If you have a keyway, why would you also want an interference or
transition
fit on a pulley?

Both. You'd want to choose the fit according to your needs, _then_
worry about whether there's a keyway to lock it against rotation.
Whether there's going to be a keyway or not doesn't change the fit
needed.


Andy

Thanks for comprehensive response.
A bit of clarification - we are not talking about a mass production
environment here, these are 1-off or maybe 6-off max per project.
The two applications I am considering currently are both chain drives.
The pulleys wouldn't need to be particularly accurately located on the shaft
I wouldn't have thought as the chain would take up a few thou' of
misalignment? The shaft sizes are 60mm and 200mm for the different
applications and (obviously) carrying some serious torque.

Any further comments/observations appreciated.

Phil

"TheScullster" wrote in message
...
Hi all

Not DIY at all really, but knowing the group's expertise in mechanical
engineering.......

If you have a keyway, why would you also want an interference or
transition fit on a pulley?
The drive suppliers we use are offering gearboxes with m6 tolerance output
shafts.
This gives a tolerance that has an upper limit larger than the nominal
bore size.
This implies to me that a pulley fitted to this shaft will tend to have at
least a possible interference on the shaft rather than being a free fit
(because the standard hole tolerances start at the nominal size and go
larger).

Am I missing something here - is this standard practice to maybe have to
lump hammer a pulley on?

No. Any pulley on an automobile crankshaft would be a sliding fit if there
is a keyway to locate it.
--
Dave Baker - Puma Race Engines

On 3 Oct, 16:29, "Dave Baker" wrote:
"TheScullster" wrote in message

...





Hi all

Not DIY at all really, but knowing the group's expertise in mechanical
engineering.......

If you have a keyway, why would you also want an interference or
transition fit on a pulley?
The drive suppliers we use are offering gearboxes with m6 tolerance output
shafts.
This gives a tolerance that has an upper limit larger than the nominal
bore size.
This implies to me that a pulley fitted to this shaft will tend to have at
least a possible interference on the shaft rather than being a free fit
(because the standard hole tolerances start at the nominal size and go
larger).

Am I missing something here - is this standard practice to maybe have to
lump hammer a pulley on?

No. Any pulley on an automobile crankshaft would be a sliding fit if there
is a keyway to locate it.
--
Dave Baker - Puma Race Engines- Hide quoted text -

- Show quoted text -

Dont bugger about - use a Taper-lock bush with suitable pulley (as
most maintenance engineers will opt for given the choice).

On 3 Oct, 14:09, "TheScullster" wrote:
"Andy Dingley" wrote

If you have a keyway, why would you also want an interference or
transition
fit on a pulley?

Both. You'd want to choose the fit according to your needs, _then_
worry about whether there's a keyway to lock it against rotation.
Whether there's going to be a keyway or not doesn't change the fit
needed.

Andy

Thanks for comprehensive response.
A bit of clarification - we are not talking about a mass production
environment here, these are 1-off or maybe 6-off max per project.
The two applications I am considering currently are both chain drives.
The pulleys wouldn't need to be particularly accurately located on the shaft
I wouldn't have thought as the chain would take up a few thou' of
misalignment? The shaft sizes are 60mm and 200mm for the different
applications and (obviously) carrying some serious torque.

Any further comments/observations appreciated.

Phil

You do not quote torque/hp but at these shaft sizes it`s obviously
considerable.In addition to the torque you have to allow for the
service factor in deciding what sort of fit you require.As you appear
to be out your depth in this you should ask the transmission people
who are supplying the sprockets for advice.At the very least you need
to use taperlock bushes.A sliding fit relying on the key to hold it
won`t last very long before the shaft,key and sprocket are worn and
needing replaced.
The reason the gearbox manufacturer works to that tolerance is that he
has to tie his sizes down somewhere.The fit of the bore to his shaft
is up to the assembler who will decide whether to specify a press fit
or use one of the many products available to obtain the same result as
a press fit.
Remember also that a 200mm bore will not be a sliding fit on a 200mm
shaft,in fact it will take a lot of heat or pressure to make it fit at
all.
Mark.
www.ems-fife.co.uk

On 3 Oct, 13:43, The Natural Philosopher wrote:
Andy Dingley wrote:
On 3 Oct, 11:29, The Natural Philosopher wrote:

A key can shear under a jerk..

sigh, no I can't be bothered.

No? I have had one do it.

Force fit on a lawnmower disc. Got loose, hit a stone, and hey presto.
Disk spinning freely on crankshaft, and key sheared off. Better than a
new crankshaft I suppose.


Rather like the shear pin on an outboard motor propellor! Perhaps
designed for the same reason?

"Dave Baker" wrote in message
...

"TheScullster" wrote in message
...
Hi all

Not DIY at all really, but knowing the group's expertise in mechanical
engineering.......

If you have a keyway, why would you also want an interference or
transition fit on a pulley?
The drive suppliers we use are offering gearboxes with m6 tolerance
output shafts.
This gives a tolerance that has an upper limit larger than the nominal
bore size.
This implies to me that a pulley fitted to this shaft will tend to have
at least a possible interference on the shaft rather than being a free
fit (because the standard hole tolerances start at the nominal size and
go larger).

Am I missing something here - is this standard practice to maybe have to
lump hammer a pulley on?

No. Any pulley on an automobile crankshaft would be a sliding fit if there
is a keyway to locate it.

BTW, you should also be aware that the key is not usually there to drive the
pulley but to locate it in a fixed orientation such as for ignition timing
purposes on an engine crank pulley. The vast majority of the drive force in
most applications is provided by friction between the pulley and the flange
on the shaft it abuts that is applied by the nut or bolt holding the pulley
on. However the key provides a useful backup for light use in case the nut
comes undone. If there is no purpose in the pulley being in a fixed
orientation there is no need for a key.

When very high drive loads are encountered, such as the axles on a car, then
the shaft is usually splined. Providing drive force via an interference fit
is very unusual and not sensible unless the axle is a very large proportion
of the pulley diameter. No matter how tight the fit on a small axle it won't
apply very much rotational torque and as soon as it slips once the system is
buggered due to wear.

However sometimes keys or pins are indeed meant to provide drive force and
in such cases are often sacrificial so they will break before something else
more critical does.
--
Dave Baker - Puma Race Engines

Dave Baker wrote:
"Dave Baker" wrote in message
...
"TheScullster" wrote in message
...
Hi all

Not DIY at all really, but knowing the group's expertise in mechanical
engineering.......

If you have a keyway, why would you also want an interference or
transition fit on a pulley?
The drive suppliers we use are offering gearboxes with m6 tolerance
output shafts.
This gives a tolerance that has an upper limit larger than the nominal
bore size.
This implies to me that a pulley fitted to this shaft will tend to have
at least a possible interference on the shaft rather than being a free
fit (because the standard hole tolerances start at the nominal size and
go larger).

Am I missing something here - is this standard practice to maybe have to
lump hammer a pulley on?
No. Any pulley on an automobile crankshaft would be a sliding fit if there
is a keyway to locate it.

BTW, you should also be aware that the key is not usually there to drive the
pulley but to locate it in a fixed orientation such as for ignition timing
purposes on an engine crank pulley. The vast majority of the drive force in
most applications is provided by friction between the pulley and the flange
on the shaft it abuts that is applied by the nut or bolt holding the pulley
on. However the key provides a useful backup for light use in case the nut
comes undone. If there is no purpose in the pulley being in a fixed
orientation there is no need for a key.

When very high drive loads are encountered, such as the axles on a car, then
the shaft is usually splined. Providing drive force via an interference fit
is very unusual and not sensible unless the axle is a very large proportion
of the pulley diameter. No matter how tight the fit on a small axle it won't
apply very much rotational torque and as soon as it slips once the system is
buggered due to wear.

However sometimes keys or pins are indeed meant to provide drive force and
in such cases are often sacrificial so they will break before something else
more critical does.

This post reminds me of he long diversion taken by Bill Bryson into the
nature of wild bears n his book 'a walk in the woods'

As he says after a chapter on his researches.

"The end result of my lengthy research was the fact that mostly bears
don't attack. But sometimes they do"

:-)

On 3 Oct, 14:09, "TheScullster" wrote:

Any further comments/observations appreciated.

Same as before- check MH for details, but a looser tolerance on a
shaft still fits into the matching loose tolerance on a hole. AFAIR,
You might need an M5 fit class rather than an M6 if there's
appreciable side-load, but you didn't mention the diameter of the
shaft anyway. M6 is the "usual" fit for motor shafts an inch or two
diameter.

I don't think you'd need any better location here than a transitional
fit and a key. If the shaft is appropriately big, then taperlocks are
another answer, assuming that the sprocket supplier offers a suitable
version. if both parts already have keyways though, I'd just use a key.