Press fit curve analysis of two different bushings (force versus displacement)

[florian245]

TL;DR Summary

Help me analyse what's wrong with my press fit curve

Hello everyone

I intent to resolve this issue for some days already but i can't figure out what's wrong

At this moment i use a kind of bushing which give me a curve like this :

As we can see I have between 46 and 47mm my effort reaching a constant value of effort for the 3mm of fitting that I have and when it reach the end there is the vertical curve. Until there it's alright

but....

I changed my supplier for my bearing and now I've curves like this

That is to say way more versatility and rather than it remains constant between 47 and 50mm I've a lot of variations and most of time a loss of effort

To precise :

My outside diameter is the same of 17,57 mm between the 2 bushings
I checked rugosity and that's alright
I polished my metal and doesn't change anything
I put my bushing in a 3d machine metrology
I looked at the CCPU details
Also the inside diameter

Does anyone have suggestions of this loss of effort ?

Thank you !!!!

 

[Baluncore]

Welcome to PF.

Does anyone have suggestions of this loss of effort ?

Yes, but I have insufficient details of the components, so must be general.

Assume the outer is thin walled, while the inner is a rigid plug, with a minimal rounded leading edge.
Assume for simplicity that the outer sleeve remains cylindrical or conical, (straight edge).
Initially, the outer becomes more steeply conical as the plug enters and the sleeve expands.
As the plug passes 46.5 to 47.1 mm, the conical sleeve gradually tips to become a parallel cylinder again.
The sleeve has been stretched, and has then tipped, so then only friction remains.

Now imagine the alternative or reciprocal situation, where a hollow insert has a flexible straight outer surface, and the surrounding sleeve is rigid and fixed.

I would expect that sort of response, if the plug or sleeve was fabricated from a composite, with a diagonally cross-laid fibre material. There will come a point when the tension in all the fibres equilibrates.

 

[florian245]

Hello thank you for your answer, clearly between 46.5 and 47.1mm it's this but the main point remains the loss of effort that exists in some cases and not others.

I give you some more details, my shaft is the 15 mm diameter hole in this following part (because the one of 14.5mm is for a smaller bushing

Later this part is transformed like this and the bearing which doesn't work is the big one

The size of the bushing is the following one

The bushings are mounted with this method :

My material is the following one :

So as we can see the loss of effort is strange while for other bushings it doesn't loss effort

Tell me if you have any more suggestions with this

Thank you a lot

Have a good day

 

[Baluncore]

The bushing is opening the hole from 17.35±0.1 mm to 17.57±0.02 mm.
There is the 4° taper over 1.5 mm shown. That change in diameter is 0.21 mm.
Since 17.35 + 0.21 = 17.56 mm, we can assume that the main part of the bushing is parallel. But we see the force increasing gently, so it may be a long taper.

There is no specification if the bushing is cylindrical or has a taper for the last 4.5 mm of movement. The earlier bushings might have been tapered, while the new ones are parallel, or possibly negative tapered over the last 4.5 mm. That would explain why the final force might fall.

 

[florian245]

As you can see the profile of my bushing is perfectly cylindrical. until 0.001mm almost

So it would be another issue
I will try to increase the size of my part hole diameter

 

[Baluncore]

Your detail 11, Radius 0.2 mm;
That provides a high point which will cause a reduction in pressure beginning over only the last 0.4 mm of the assembly.

What is the thickness of the plate where it has the 15.0±0.5 mm hole?
I think that is the clue to where and how the bushing distorts during assembly.

Maybe the yield point is different between the earlier and later supplier's bushings?
Check the heat treatment and the thermal profile during machining.

The tight part of the assembly will be moving up the 4° taper over 1.5 mm travel.
Once the plate moves halfway past the top of that rise, the bushing will rock on the crest, from conical to cylindrical, as it redistributes the internal compressive forces. That will show an even or loss of force required over the last half-plate-thickness of travel.

 

[florian245]

Sure for the detail 11 but the loss starts way before 0.4mm

The thickness is about 4 mm.

The yield point is upper than before

Yes clearly but the 4° taper starts 6 mm before the end of the assembly and the loss of effort is 1.5mm only before the end of the assembly.

 

[Baluncore]

Yes clearly but the 4° taper starts 6 mm before the end of the assembly and the loss of effort is 1.5mm only before the end of the assembly.

It is where the 4° taper ends that is important, and how does the bushing change from conical distortion, back to cylindrical, as it crosses that tipping point, 2 mm before the end.