The manufacture of bearing balls

There is a further practical problem for ball bearings and that comes from the methods used to manufacture the balls. Most metal artefacts are made by machines that cut or grind and usually the work piece is held in a chuck or a vice or some special fitting whilst the cutting is performed. The trouble with making balls is that there is no way to hold the ball to work on it and balls are required by the thousand. Steel balls are made in an unlikely way that may go back for centuries to the manufacture of spherical marble balls for architectural use. The method that is actually used does not involve holding the ball but employs a random process that produces balls that are very nearly spherical but in fact are the outcome of chance.

 

I once came across the process in action in a river. At a weir there was a standing wave at the foot of the weir wall. By chance a block of wood from the trunk of a tree about 500 mm in diameter and about 500 mm long had become “caught” in the back roll. It could never get through the standing wave as it went through a cyclic motion of being carried under at the weir wall bobbing up due to its buoyancy and then back-rolled on to the wall by the wave and thrown back to the weir wall. It was getting battered on the wall and the apron. When I saw it, it was two thirds of the way to becoming spherical.

 

Bearing balls are made from steel wire that is cropped accurately to a required length and them formed into a ball by sudden squeezing between two hardened cups. This produces a blank ball that is slightly over size and has a small flash all round it. Hundreds or thousands, according to size, of such ball blanks are fettled to get rid of the flash. Then the balls are fed continuously and repeatedly through a ball-making machine. This machine looks remarkably like a machine for milling flour in that it has two cast-iron, cylindrical plates of about the same proportions as millstones. One is flat and the other has circumferential grooves as shown in figure 16-29. They are set up so that the flat plate would rotate anticlockwise if fed as in figure 16-29. The balls are fed into the open segment with a water slurry and travel round the grooves colliding with each other and the solid surfaces and gradually being worn to a spherical shape. The balls make many passes through the grooves and come out looking remarkably spherical.

 

There is a great deal more to the process including hardening etc. but our interest is in the fact that this is a random process.

 

In practice the balls resulting from the manufacture of a batch will vary slightly in shape. One variation will in deviation of the surface from a true sphere because of tiny surface imperfections.  Then if one could imagine this surface having some mean shape it might not be quite spherical and it will certainly vary slightly in mean diameter. This is hardly surprising.

 

It is normal to grade the balls from a batch and use the best for top quality bearings and the worst for cheap bearings for which there are many applications. Other grades fit between.

 

The races by comparison with the balls can be made very accurately because they can be held and their shapes generated on machines.

 

The net result is that ball races of ordinary standard have clearances that vary somewhat from ball to ball and this means that the design clearance must be chosen to accommodate the largest ball that is likely to be inserted and that the smallest ball will have greater clearance than is desirable. Only the very best ball races will have uniform clearance.

 

The object of designing a ball bearing is to support a shaft so that its axis of rotation remains stationary relative to the bearing block. We have seen that a ball bearing with perfect balls but with clearance will not achieve this if it is dry. However if the bearing is lubricated with oil or grease this problem can be reduced to the point of being eliminated.