Photo: Steve Snodgrass (Flickr)
Machines wear out or jam because material is removed from one solid surface as it slides over another.
Adhesive Wear Removal
The most common way material can be removed is by adhesive wear, which happens because every surface has bumps and valleys, no matter how small.
When a tiny bump on one surface touches a tiny bump on another, the two surfaces may become welded together over a microscopic area.
Resistance And Friction
If some force continues to slide one slab of material over the other, then that microscopic weld will break. The material resists being broken, of course, and we feel that resistance as friction.
When the weld does break, the break may not be at the original junction. A piece of material from one surface ends up stuck to the other surface. Eventually that piece is broken off the new surface. Meanwhile, all this welding and breaking causes the kind of random motion of atoms we feel as heat.
Two Things May Happen
If the space between the sliding surfaces is big enough, that loose piece may be simply pushed out, leaving the surfaces still sliding but with more space between them because material has been removed. Eventually the two parts don’t fit as tightly as they used to, and we say the machine is worn out.
On the other hand, if the space between the sliding surfaces is too small for wear particles to move around freely, then those particles prevent further sliding motion and we say that the machine has seized or jammed
Lubricants reduce adhesive wear by keeping sliding surfaces apart and carrying wear particles away.
There are other kinds of wear besides adhesive wear — for instance, there is abrasive wear, which occurs when hard particles gouge a softer surface.
The oil filter in a car helps prevent abrasive wear by catching hard particles before they get between sliding metal surfaces.