Recommendation for tap fit hole tolerance in 4140 pre-hard steel
 

If it is that RC28 pre hardened from J&L, and old tap should cut it.

Recommendation for tap fit hole tolerance in 4140 pre-hard steel
 

On Nov 29, 10:01 pm, Clark Magnuson wrote:
If it is that RC28 pre hardened from J&L, and old tap should cut it.

Hi Clark,

Please let me clarify my question, when I said "tap fit" I meant that
the pin will be tapped with a hammer through the hole in the housing.
There are no threads involved.

My concern is with the min. and max. hole size that will allow the pin
to be lightly hammered through the housing hole fairly easily, but I
don't want the housing rotating on the pin. The housing and pin must
rotate together, and the ends of the pin rotate in needle bearings.

Sorry for any confusion.

Thanks
John

Recommendation for tap fit hole tolerance in 4140 pre-hard steel
 

John2005 wrote:
On Nov 29, 10:01 pm, Clark Magnuson wrote:
If it is that RC28 pre hardened from J&L, and old tap should cut it.

Hi Clark,

Please let me clarify my question, when I said "tap fit" I meant that
the pin will be tapped with a hammer through the hole in the housing.
There are no threads involved.

My concern is with the min. and max. hole size that will allow the pin
to be lightly hammered through the housing hole fairly easily, but I
don't want the housing rotating on the pin. The housing and pin must
rotate together, and the ends of the pin rotate in needle bearings.

Sorry for any confusion.

Thanks
John


Sorry, I screwed up.
It seems to me that for a long pin, the calculation on the pin would be
bulk modulus, and the calculation on the hole would be Lame's formula.

The equilibrium between those two forces times the co efficient of
friction would give the force needed to drive in the pin.

The heat treat on the hole would make the math the same up to the point
that the hole permanently stretched. The hole when heat treated could
stretch larger and still spring back. The heat treat would then be a
good thing, if the forces are not so high that you can't get the pin in.

Recommendation for tap fit hole tolerance in 4140 pre-hard steel
 

On Nov 30, 10:07 am, Clark Magnuson wrote:
It seems to me that for a long pin, the calculation on the pin would be
bulk modulus, and the calculation on the hole would be Lame's formula.

The equilibrium between those two forces times the co efficient of
friction would give the force needed to drive in the pin.

The heat treat on the hole would make the math the same up to the point
that the hole permanently stretched. The hole when heat treated could
stretch larger and still spring back. The heat treat would then be a
good thing, if the forces are not so high that you can't get the pin in.

Whack the hole with a ball pein hammer on the exit side and let the
pin stretch it back.

Recommendation for tap fit hole tolerance in 4140 pre-hard steel
 

Thanks for the feedback guys,

"Mechanical Magic" from the Sci-eng group gave me the following from
Machinery's handbook...

Machinery's Handbook 19th calls for a shaft of 1/4-13/32 to be:
"Easy Driving Fit" to be .00028-.00047 interference.
"Close Driving Fit" at .00047-.00083
"Forced Fit" .00083-.00146

I can rig up a small press for this thing but it's still best to have
the easiest press possible.

As far as the bearings at each end, they are just drawn cup needle
rollers with no inner ring. The rollers roll right on the shaft. I
will slide the dowel through the bearing ID and then press or lightly
hammer the pin into the 4140 housing and on through to the other
bearing. I will probably start a pin in at the other end just a little
to align, and then the pin I'm driving in will push the other pin out
when it gets through.

Thanks again,
John

Recommendation for tap fit hole tolerance in 4140 pre-hard steel
 

Your fit will depend also on how you machine the bores - i.e. the
finish. Bored, reamed, honed, burnished? A roughly bored finish with
little interference might give you a light press fit, whereas a bore
burnished to the same size might be a heavy press or shrink fit.

There's always Loctite.....

John Martin

Recommendation for tap fit hole tolerance in 4140 pre-hard steel
 

On Nov 30, 5:18 pm, John Martin wrote:
Your fit will depend also on how you machine the bores - i.e. the
finish. Bored, reamed, honed, burnished? A roughly bored finish with
little interference might give you a light press fit, whereas a bore
burnished to the same size might be a heavy press or shrink fit.

There's always Loctite.....

John Martin

Loctite is almost certain to migrate into the needle bearings. When I
wrote 'Whack it with a hammer' I meant swage the exit hole down a
little and let the pin expand it back to a very snug press fit but
that might score the needle bearing journal and it wouldn't stay tight
for long under a load.

I've had some success adjusting a press fit without precise diameter
control by not reaming full depth, so the interference fit section is
short. If it's too tight I just ream a little deeper and try again.
This is the prior-planning version.

Move the needle bearings into the housing, if there's room.

Change to a drillable pin, install a roll pin in the shaft and
matching slot on one side of the housing and a snap ring on the other,
assuming you have enough room between the assembled 4140 housing and
the bearing cup to push in the roll pin and install the snap ring.
Lots of low-end equipment uses mild steel for roller and needle
bearings.

Press the needle bearing cups in last. Then you could clean Loctite
off the dowel pin. If you can tolerate heavy grease it will keep the
needles in place pretty well, otherwise put in a temporary plug.

Recommendation for tap fit hole tolerance in 4140 pre-hard steel
 

Thanks for the additional info Jim

I had considered moving the needle bearings into the housing, but It
seemed that when pressing the pin through the upright support and then
into the bearing bore, I would not be able to feel if it were going
into the bearing or perhaps hurting the bearing. If the bearings are
pressed into the supports first, then I can slide the pin through the
bearing ID, feel it start in the housing hole, then push it through to
the other side. I was going to start a pin just a little on one side
for alignment, then press another pin in from the other side, the
second pin will push out the first pin as it goes through.

If I don't put the bearings in the housing, I like your suggestion to
press the bearings in last. This way I could press the pin through and
not worry about hurting the bearings, and then press the bearings into
the upright supports last. The full complement needles are retained by
the ends of the cup, so theres no problem with them coming out. I have
to use a hard shaft for this application so that's why I'm sticking
with a dowel pin, it comes in the exact length and OD I need, plus
their fairly cheap.

Thanks again,
John