friction factor 2

[SimonJ]

I am trying to accuratly determine the axial force in a screw by knowing the torque applied to it. The screws are very small M1.6 - M3 so an ultrasound method is no good for determining the stress in the screws.
Is there are more accurate method of torque control than a torque wrench for such small screws?
Does anyone know the friction factor between a stainless screw and an aluminium casting, or where to find it?

Cheers

 

[boo1]

The frictional coeficients typically used are:
0.130 Stainless or acid resistant steel external/internal threads, molybdenum disulphide lubricated.
0.168 Stainless or acid resistant steel external and internal threads, waxed.
0.125 No finish on steel external thread, no finish on steel internal thread, no lubricant.

The tightning factors are a result of the tightning method used. The followin are typical values:
1.1 Yield Controlled Tightening.
1.1 Angle of rotation controlled tightening.
1.2 Tightening by measurement of the elongation of a calibrated bolt.
1.4 Torque controlled tightening using dynamic torque measurement. Experimental torque value - small scatter.
1.6 Hydraulic tightening with control by length measurement and/or pressure measurement. Short bolts.
1.6 Torque controlled tightening using dynamic torque measurement. Experimental torque value.
1.6 Torque controlled tightening using dynamic torque measurement and precision tools. Torque value theoretical.
1.7 Torque controlled tightening using a nutrunner. Torque determined theoretically. Tool calibration tested.
1.8 Torque controlled tightening using a signalling or automatic tripping torque wrench. Torque value theoretical.
2.5 Torque controlled tightening using a nutrunner. Torque determined theoretically.

Some designers do not like to use the SST fastener into a Aluminum casing

Hope this helps Boo

 

[AJohnson]

Hey Boo,

The third value you posted...is that saying steel on steel with no lublication is 0.125? That is what it appeared to say to me and as such, seemed quite wrong. My reference states steel on steel with no lubrication has a static friction factor of 0.6 and a kenetic friction factor of 0.4. Since you didn't state it explicitly, are the values you posted static or kinetic? Comparing them to values in my reference, they appear to be static friction factors.



Andy

 

[boo1]

You are measuring a installation torque (dynamic). The beak away torque (removal) would require the static value.

 

[AJohnson]

Boo,

I understand dynamic (kinetic) and breakaway (static).

You still haven't addressed steel on steel with no lubricant friction factor being lower than steel on steel WITH lubricant. That can't be right...or else what is the purpose of lubricant?...to INCREASE friction?...I don't think so! Andy

 

[boo1]

My understanding of the lubricant or coating materials used on fasteners with the intention of reducing the scatter in the thread and bearing surface friction coefficients. The factor I provided was from mean reported factors. These are just examples. The best method is to dirive you own values experimentally.

A major problem with using the torque control method to indirectly control the clamp force in the fastener is that only a small proportion of the torque is actually used to extend the bolt. The majority is absorbed by friction between the nut face and the joint and in the threads. Some fasteners use a nylon insert or have a distorted thread so that a torque is required to run the nut down an unclamped bolt thread. Interestingly, for the same stress in the bolt, the required tightening torque between the prevailing and non-prevailing bolt types varies by less than 4%. The prevailing torque type nut however results in significantly less preload (16%).