Hi all. I see plenty info here for bearing and bush interferance fits but the bush can shrink a little if needed. I am over boaring a hydraulic ram pin boss to 100mm and would like to get a nice press fit not like the original manufacturers sloppy loose fit that caused the problem in the first place. Is 0.1 mm too tight?
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Pin bore interference fits
- Thread starter Volvomad
- Start date
funwithfuel
Senior Member
That sounds to be right. .003" for up to 6" is what I remember. That would be your .1mm up to 150mm. Beyond that there's a machinists rule to follow, but I don't remember the proportion.
Good luck stuffing that pin. Dry ice would be a great help
Good luck stuffing that pin. Dry ice would be a great help
funwithfuel
Senior Member
The freezer has never worked for me, not on 'steel' , besides, I love the way she moans and cries when I shove her in them cold pellets
Use those leftovers to beat that pin in.
terex herder
Senior Member
1 part in 1000 is a standard press fit. Steel grows 6 parts in 1,000,000 per degree F. So a 167 degree F temperature difference will set your interference to zero. Double the temperature difference if possible and be ready with a big hammer. As long as the big hammer and appropriate punch are at hand they won't be needed.
Nige
Senior Member
You're in an agricultural area. Ask any of your farmer friends who have cattle where their supplies of liquid nitrogen (used for freezing semen used in artificial insemination) come from. A good soak in LN2 and the bush will fall into the hole so fast you'll need to be quick to stop it falling out of the other side.
Failing the above an ice cream factory can usually supply dry ice. That's a pretty good substitute.
.RC.
Senior Member
This is all you need. At 0.1mm you are into tight shrink fit. I would probably aim for 0.05 - 0.07 myself.
Ice cream and AI are out of season unfortunately. A tank of cold stuff would be very handy but I wouldnt use it often enough. The domestic freezer wont do much but every little helps (I think). Thanks for the chart. I will post a pic if it gets stuck and I have to gas it out.
OzDozer
Senior Member
Grab a Carbon Dioxide fire extinguisher and squirt the bush with it. Instant dry ice.
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Entropy1
Well-Known Member
I second the above recommendations.
Liquid nitrogen is by far the best for hand-assembling interference joints, however most vendors refuse to sell liquid nitrogen unless you've got a proper transport-rated cryo-flask - which are not cheap $$$.
An alternative: some grocery stores sell dry ice in 10-ish pound blocks. Ultimate weight depends on how much has sublimed prior to purchase. It's usually sold by the pound at time of purchase. While not as cold as nitrogen, dry ice is often adequate to get the job done.
Example: if your pin is precisely machined to 4.0000" diameter at 68 degrees F, the pin will shrink to 3.9954" diameter when packed within dry ice (aka -109 degrees F). A styrofoam cooler with pin & dry ice is all you need. And maybe some well insulated leather gloves.
Note: the same pin bathed in liquid nitrogen will shrink to 3.9899" diameter (-320 degrees F)
The other thing you must be careful about - both dry ice and liquid nitrogen will cool most steel alloys to well below their respective brittle-to-ductile transition temperature. This means do not subject the parts (pins or bushings) to impact while they're cold, because you can crack them like glass.
Liquid nitrogen is by far the best for hand-assembling interference joints, however most vendors refuse to sell liquid nitrogen unless you've got a proper transport-rated cryo-flask - which are not cheap $$$.
An alternative: some grocery stores sell dry ice in 10-ish pound blocks. Ultimate weight depends on how much has sublimed prior to purchase. It's usually sold by the pound at time of purchase. While not as cold as nitrogen, dry ice is often adequate to get the job done.
Example: if your pin is precisely machined to 4.0000" diameter at 68 degrees F, the pin will shrink to 3.9954" diameter when packed within dry ice (aka -109 degrees F). A styrofoam cooler with pin & dry ice is all you need. And maybe some well insulated leather gloves.
Note: the same pin bathed in liquid nitrogen will shrink to 3.9899" diameter (-320 degrees F)
The other thing you must be careful about - both dry ice and liquid nitrogen will cool most steel alloys to well below their respective brittle-to-ductile transition temperature. This means do not subject the parts (pins or bushings) to impact while they're cold, because you can crack them like glass.
Entropy1
Well-Known Member
Just for chit-chat – Lets say you have piece of 2” thick steel plate. You drill a 4” diameter hole into the plate, and you wish to insert a 4.002” diameter pin (for a 0.002" interference fit).
Cooling the pin with dry ice (-109 degrees F) will shrink it diametrically by 0.0048”, providing an overall insertion-clearance of 0.0028”. That’s not a lot. Obviously the bore must be completely free of any nicks, or burrs, or debris (hone it – clean it well). The pin must be in the same condition, or you will not be pushing the pin into the hole by hand. You must go quick – before it starts to adsorb heat.
Once inserted to a depth of 2 inches, and after the pin achieves room temperature, there will be approximately 7200 psi of compressive stress generated against the surface of the pin. Assuming a 0.2 friction-coefficient (typical for steel against steel), it will take approximately 36,191 pounds of axial force to push the pin from the hole. Or it would take approximately 6,032 foot-pounds of torque to cause the pin to rotate within the hole.
Why is this important? Let’s say your plate was 3 inches thick and you only got the pin interred 2” deep before it warmed up. Better have a big hydraulic press. The insertion force required at a depth of 3 inches is about 54,287 pounds. Required torque to cause rotation 9,408 ft-lbf.
And if you increase the diameter of the pin to 4.003” (an extra thousandths of interference). The insertion force required to achieve a depth of 3 inches becomes 81,430 pounds axial. Required torque for rotation becomes 13,572 ft-lbf.
However when the pin is properly chilled, you can insert & rotate by hand. . . . Obviously with heavy interference fits, it becomes clear why liquid nitrogen is the preferred cooling method. . .
Cooling the pin with dry ice (-109 degrees F) will shrink it diametrically by 0.0048”, providing an overall insertion-clearance of 0.0028”. That’s not a lot. Obviously the bore must be completely free of any nicks, or burrs, or debris (hone it – clean it well). The pin must be in the same condition, or you will not be pushing the pin into the hole by hand. You must go quick – before it starts to adsorb heat.
Once inserted to a depth of 2 inches, and after the pin achieves room temperature, there will be approximately 7200 psi of compressive stress generated against the surface of the pin. Assuming a 0.2 friction-coefficient (typical for steel against steel), it will take approximately 36,191 pounds of axial force to push the pin from the hole. Or it would take approximately 6,032 foot-pounds of torque to cause the pin to rotate within the hole.
Why is this important? Let’s say your plate was 3 inches thick and you only got the pin interred 2” deep before it warmed up. Better have a big hydraulic press. The insertion force required at a depth of 3 inches is about 54,287 pounds. Required torque to cause rotation 9,408 ft-lbf.
And if you increase the diameter of the pin to 4.003” (an extra thousandths of interference). The insertion force required to achieve a depth of 3 inches becomes 81,430 pounds axial. Required torque for rotation becomes 13,572 ft-lbf.
However when the pin is properly chilled, you can insert & rotate by hand. . . . Obviously with heavy interference fits, it becomes clear why liquid nitrogen is the preferred cooling method. . .
1693TA
Senior Member
Had a guy want to give me a dewar at one time but I didn't really see a need, (at the time) but have wisen'd up a bit over the years. Another of life's lessons ya got to learn along the way I suppose.
Nige
Senior Member
What I was thinking was maybe someone somewhere has a Dewar flask you could borrow/rent to pick up enough LN2 for the job you plan on doing. if it's not the season that could be a perfect opportunity because the owner would not be using it. By the sounds of it you're going to need at least a couple of gallons.
terex herder
Senior Member
Dry ice is much more effective in a bath of acetone, but never stick your finger in a dry ice/acetone bath. The frostbite is immediate and severe, much worse than liquid nitrogen, even though the liquid nitrogen is much colder. I can pick small parts out of liquid nitrogen with my bare hands, but would never try it with dry ice and acetone.
One problem with cold shrink fits is frost/ice accumulation. Depending on dew point, it can become quite severe. When the situation permits, heating the part is, to me, preferable to cold shrink. A warmed part can also help with ice accumulation on cold shrinks. Every shop has the equipment to heat a part.
One problem with cold shrink fits is frost/ice accumulation. Depending on dew point, it can become quite severe. When the situation permits, heating the part is, to me, preferable to cold shrink. A warmed part can also help with ice accumulation on cold shrinks. Every shop has the equipment to heat a part.
