Press fit bushing, How do I calculate bushing ID after press fit 1

[kurtmcm]

I am trying to calculate how much a steel bushing that is press fit into a steel housing will shrink on the ID so that i can tolerance the shaft that goes through it properly. I have not had any luck finding this calculation.

Thanks,
Kurt

 

[Heckler]

Google Hoop Stress calculations. Also, if this is rotating equipment you will have to take into count the thermal expansion of both materials.

Heckler
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[kurtmcm]

Thank you for that i am looking into hoop stress. which leads me to my next question.

In the calculation to determin the pressure between the bushing and the bore there is a variable for the radius after it is pressed into the bore. The book i have is assuming that the bore will move the same amount as the bushing wall (the reference is for thick walled cylinders) but what if the bushing wall is thin and the housing of the bore is very thick. It seems to me that the bushing would shrink more than the bore would expand. This would change my radius after assembled and the answer to the stress equation.

Is this a bad assumption? Can it be calculated differently?

Thanks,
Kurt

 

[sbozy25]

Well this could be rather simple. If this is a split ring bushing you can just take your ID of the hold plue the 2xthickness of the bushing and you will have your new ID.

This is an application that automotive uses all the time in steering columns. We would press a split ring steel bushing into a houseing and then put a bolt through that. 99& of the time your thermal expansion is negligible. I might also reccoment if you are using this in a rotation situation, use a steel mesh bushing with a PTFE coating to reduce friction. you will save your self a bunch of head aches.

 

[kurtmcm]

Thanks for the idea of a split bushing. The one in this application is solid and hardened to Rc 58-64. (not sure how the hardness plays into the equation or if it does)

Thanks,
Kurt

 

[Heckler]

But remember if you're using a metal backed split bushing like these

http://www.afmusa.com/bushings.metalbacked.asp

The position of the split is critical in some applications.....ie high load & speed. We have used these types of DU Bushings on a number of pumps.

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 3.0 & Pro/E 2001
XP Pro SP2.0 P4 3.6 GHz, 1GB RAM
NVIDIA Quadro FX 1400
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(In reference to David Beckham) "He can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. Apart from that, he's all right." -- George Best

 

[Heckler]

One more thing regarding these bushings is they come with either a low percentage of lead or no lead. If this is an application that is around food or other human consumption items then no lead or if you're selling to the EU.

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 3.0 & Pro/E 2001
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NVIDIA Quadro FX 1400
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(In reference to David Beckham) "He can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. Apart from that, he's all right." -- George Best

 

[rb1957]

you're right, kurtmcm, the final diameter (and the interface pressure) is a function of the interference. Timoshenko (Plates and Shells) does a good analysis of this, it depends on the stiffness of the bushing compared with the stiffness of the parent material. It may be an iterative solution ... how much pressure applied to the bush to shrink the OD by 0.001", how much pressure applied to the hole (in the parent material) to expand the hole dia. 0.001" ... when the pressure applied to both causes a compatable change, you've got your answer

 

[kurtmcm]

Gentalmen,

This is the application.

I have a steel pin going through a steel housing. the end of the pin is seeing a load that is perpendicular to the axis of the pin and the bore is wearing out. the pin does not rotate or slide in the bore. The addition of the new hardened steel bushing is to strengthen the bore so it does not wear out due to the over hung load. i am actually using the inner race of a needle bearing as the bushing.

We want the bushing to be Press fit and the pin to be a tap of tight slip fit.

Thanks kurt

 

[kurtmcm]

Thankyou rb1957

I'll try that

 

[rb1957]

when you say the bushing is the inner race of a needle bearing, are you putting a needle bearing into the hole ? I don't think so, but I think you're maybe being a little short-sighted by grabbing wahtever is close to hand and hopig it'll work ... it might, but it probably won't.

 

[TheTick]

I find that "conservation of volume" works very well for finding final ID of a bushing. If the housing is large engough, the housing defelection is negligible. The bushing OD compresses to the ID of the housing bore. The volume (in 2D, cross-sectional area) remains the same, and can be used to calculate final ID of the bushing.

Area_before = Area_after

A = [π](r²_o - r²_i)

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[desertfox]

try this calcu

http://www.engineersedge.com/calculators/machine-design/press-fit/press-fit-calculator.htmlator:-

 

[kurtmcm]

in responce to RB1957:

No we are just using the inner race like a steel bushing. These are very hard and come with thin walls and a large variety. Why do you think this design will have problems?

Thanks,
Kurt

 

[rb1957]

kurtmcm, i just thought that an inner race of a bearing was desinged to do what it does (ie be the inner race of a bearing). I don't know bearings very well, I just thought that they won't make good bushings.

to the tick's post ... yes, a simple way to determine the radius change is to say that it equals the physical interference before installation ... you make the ID of the hole slightly smaller than the OD of the bush (or bearing), LN freezes the part, shrinking it, allowing it to fit into the hole.

 

[kurtmcm]

We also freeze some bushings to get them to fit in the hole easier.

 

[sbozy25]

Ok if it is not too late for you to change I think you should do the following. Talk to a company called "Saint Gobain" they are located in Farmington Hills Michigan. They make tefflon coated split back bushings for this exact appliation. They will press into your housing and perform excellent. I highly reccomend this. Also, they tend to have really competitive prices. You give them your application and design constraints and they can provide you with a long life product that fits perfectly every time.

If you can make a design change, you should. We had issues with bushings such as what you are using at my last place of employement. They tended to rip, and deform extensively in production. We were forced to change, and this company came to our rescue.

 

[MintJulep]

If your hole is wearing, there IS relative motion.

If you really need to tightly control the ID of a press-fit bushing, select one that is undersize, and ream after it is pressed in place.

 

[unclesyd]

Take a look at the "Tension Bushings" offered at this site. If you look at the "product specifications there are tables of dimensions that may be a help.
We used quite a number of this type bushing in some of our machinery,

http://www.rollpin.com/ProductInformation/TensionBushings/index.html#prodspecs

 

[kurtmcm]

Question for desertfox (Mechanical)

Have you used the

http://www.engineersedge.com/calculators/machine-design/press-fit/press-fit-calculator

before?

Do you know what is meant by the inerface diameter (in the outer body parameters)?

and

Do you know what is meant by the Intended outside diameter (in the inner body parameters)?

Are these the mean values of the od and id tolerances?