Interference Fit case -Assembly stack up

[filippoGDeT]

Hello Everybody,

I have some difficulties to understand how consider Shaft and Holes on GD&T Assembly stack analisys (WCC)in case I'm doing radial stack vector and the two parts are coupled with interference fit. Which dimension should I Consider? Could you kindly provide some examples?
Do you think it is feasible also include TIR deviation?

Thank you very much to who can help me.

Filippo

 

[3DDave]

With interference fit there is no contribution from clearance; since it can't be negative, the relative position of the two parts is considered to be 0.

What does need considering is that the location and orientation of one part is almost entirely based on the mating feature.

For example if a pin is pressed partly into a hole, the apparent position of the exposed end of the pin can be a exaggerated over the position of the pin because a small orientation change in the hole is carried out over the exposed length of the pin.

 

[drawoh]

filippoGDeT,

Stack analysis implies to me that you have more than two parts. An interference fit generally is two parts, only. Are you trying to press-fit multiple pins?

--
JHG

 

[filippoGDeT]

Dear Both,

Thank you so much for your very usefull reply! Just to better clarify my thought herebelow one possible simplifed scenario.
Suppose that i need to calculate a minimun distance on a gear box from one of the gear and the sorrounding surface of the box.
Suppose also that the gear is fitted ( as instance ) with the shaft by intereference. The minimun distance is influeenced by:

-The outside gear diameter
-The position of the bearing seat on the box that support the shaft
-The possible radial play of the bearing
-the position of the gear relative to the shaft

In this case if i well undesrtood, during my analisys, my stack pass directly from the true axis of the shaft from the true axis of the gear withouth considering any dimensional variation of the shaft and the ID of the mounting hole of the gear as the two parts are mounted by interference.

In others words I have to ignore any dimensional requirements in case.

I hope my example is quite clear.

Looking forward your comments,

Regards!

 

[drawoh]

filippoGDeT,

Let's do your gearbox example. You are concerned about the gear's outside diameter making contact with the gearbox housing.

What you need to do is draw a circle that encloses the maximum volume that may be occupied by the gear at any time. You can work out the gear's maximum diameter, its addendum. Then, you need to work out all the errors...

[ul]
[li]Centreing of gear teeth on bore[/li]
[li]Clearance of gear bore on shaft[/li]
[li]Centreing of gear mount face to shaft to bearing face[/li]
[li]Clearance of shaft to bearing face[/li]
[li]Clearance inside bearing and accuracy of bearing[/li]
[li]Clearance of bearing OD inside bearing mount[/li]
[li]Positional tolerance of bearing mount[/li]
[/ul]

Obviously, anything you can press fit has a clearance error of zero, and is difficult to assemble.

Each of your errors is a circle. You can add the diameters to get the total enclosed outline. You start with the maximum addendum of your gear. You add half of each clearance to the addendum, and you add each positional tolerance diameter. The result is the space that must be cleared by your housing.

Another approach is to provide a relatively gross clearance between your gear teeth and your housing.

--
JHG