Very crude values of wall friction can be determined by placing bulk material on a plate of the contact suface condition of interest and inclining until it slips. This method does not take account of any suface cohesion and more care is needed to deal with the difference between static and dynamic friction or differing contact pressures, so any serious investigation should use a controlled stroke device that allows variable normal loads to be applied and measures the force required to initiate and sustain motion. Both for convenience and to reflect common motion velocities of material in hoppers and silos, where wall friction is an important design parameter for mass flow, most wall friction measuring equipment employs a drive to advance the test material at a very slow slip velocity, typically around 2.5 mm/minute. A light, circular cell is used to contain the sample of bulk material and a hanger that allows different weights to be applied to generate a range of normal stresses embracing the contact pressure of interest. The normal procedure for hopper design purposes is to measure the force required to sustain slip at each step of increasing loads up to the maximum, and then take the values as each incriment of reducing loads during one continuous sliding period. This is the reason for using a relative long stroke machine, as stopping and starting or going back on the surface to take other emasurements will confuse the situation.

The readings are then plotted on a graph of force to sustain sliding against the siurface contact load. There is usually a small displacement between the lines of increasing and decreasing load that indicates a hysterisis effect but the slope is usually relatively straight. If the lines pass through the origin it indicates there is no cohesive attraction tot h e surface but a small intercept shows that the 'effective' wall friction for low contact stresses can be quite high.

The surface should be smooth and consistant, with note taken of the direction of any 'grain' on polished surfaces. Different contact materials may be compared and the value used for any application involving relative contact of the material on a contact surface, such as chutes, screw feeders, mixing blades and the like.

Whereas a force guage may be used for recording specific values, a continuous force recorder is necessary when exploring phenomenon such as 'stick-slip' or to highlight delicate changes of resistance.

You can see photgraphs and the specification of the Ajax wall fricting device on the web site www.ajax.co.uk/testers. Should you require any more details comment or advice please send you query to lyn@ajax.co.uk ■