Question

Why are the tolerance characteristics categorized as forced
fitting and loose can't represent shape and position tolerance?

Answer 1

Force fit suitable for parts which can be highly stressed or for shrink fits where the heavy pressing forces required are impractical.

The different types of fits are as follows.

Clearance Fit: In this type of fit the shaft diameter is always less than the hole diameter. For any hole and shaft assembly, if the upper limit size of the shaft is less than the lower limit size of the hole then that type of fit is known as clearance fit. Shaft can freely slide or rotate in the hole. Force is not required for this fit.

Interference Fit: In this type of fit the minimum diameter of the shaft is always greater than the maximum diameter of the hole. There is no relative motion between shaft and hole. Force is required for this fit (either heating the hole or freeing the shaft).

Transition Fit: If the maximum hole size is greater than smaller size of shaft or minimum hole size is lesser than maximum size of shaft, such type is called transition fit. This type of fit lies mid way between clearance fit and interference fit and it may sometimes provide clearance fit and sometimes interference fit. For may or may not be required.

Press fits rely on constant stress and friction. In steel, if you press an oversized pin into a hole, they’ll stay together indefinitely. But plastic will flow under constant strain, eventually causing the stress—and thus the friction—to disappear.

With that negativity out of the way, let’s look at the proper use of press fits. As mentioned, the assembly method relies on having two parts trying to occupy the same space. But how much interference is right?

Small interference results in enormous force. And the axial holding force is not just keeping your parts together—it’s also the force required for assembly. You’ll need to be very careful when specifying press fits or risk breaking the hydraulic press. This tight machining tolerance is also one of the primary reasons you should avoid press fits for common industrial assembly.

:Never more than two pins per assembly operation. Even better, use only one interference fit and align the parts with a slip-fit second pin. If you must use two press-fit pins, be certain to use GD&T tolerancing, with the first hole as the datum for the second hole, to minimize error between the two features.

Thermal restrictions on material:- Another salient rule of nature: Everything shrinks in the cold, but not all materials shrink at the same rate. This is important when designing press fits—only use like materials if the parts will experience temperature variation.

Let’s say you’ve used a one-inch nominal aluminum pin in a hole on a 410 stainless steel part, with 0.0007 inches of diametrical interference. How cold can it get before the shrinkage completely negates the interference?

Looking at the coefficients of linear expansion in the two parts, we can see that for every degree Fahrenheit the parts cool, the aluminum will shrink by about 0.0000125 in/in, while the steel will shrink by less than half that amount, 0.0000055 in/in. If the parts are assembled at 75 degrees, and then taken to minus 25 degrees, you’ll lose all the holding power of the press fit. So use materials with similar thermal expansion when designing press fits.

Because of gradual cutting tool wear and minute changes in the machine tool internals due to temperature changes and wear/movement of internal parts, machined items can not all be made perfectly to the same dimension.

It is permitted to make the part to within a range of sizes.

That range is called the tolerance on the dimensions.

Figure shows a dimensioned shaft and a dimensioned hole in a block with tolerances to provide a transition fit when assembled.

At the largest sized shaft and the smallest sized hole, they would contact.

This tolerance is too tight for a shaft that had to move through the hole but might be suitable for the outer race of a bearing fitted in a bearing housing of a rotating shaft.

In such a case the bearing race must not move on the shaft (spin) as it will wear the shaft, so an interference fit might be suitable.

If the load on the bearing was large, or there was a lot of vibration or the shaft was spinning very fast it would be better to make it a light interference fit.

If the shaft were large and rotating at low speed and repairs had to be done by the tradesman while in-the-field without access to bearing removal and installation equipment, it might be better if it was a tight clearance fit.

Selection of tolerances for a part is made after considering –

1.the speed at which the part moves

2.the applied loads and forces it must withstand

3.the amount of vibration permitted

4.whether grease or oil lubrication is used ease of assembly

5.changes in size due to thermal expansion.