Shygirl,
In general lockwashers are a very simple device which relies up a combination of spring tension and friction to hold a nut or bolt in place.
Common lockwashers are made of a spring steel and are cut through on one side, then bent slightly so the two edges of the cut are our of register with each other. At the torque is applied the washer is compressed flat and if the nut or bolt tries to move the cut edge binds into it by friction to prevent movement. Common lock washers are made in many differing metals, steel, carbon steel, stainless steel, brass, bronze, etc. It is always best to match all fasterners by the type of material to prevent electrolosis corrosive effects of dissimilar metals. Common lock washers come in a large variety of sizes from a minor fraction of an inch to 2" or 3". In general, common lock washers offer the best holding power but are not well suited to repeated movements or tensioning.
Internal tooth and external tooth lock washers are generally made in smaller diameters and used in light duty applications such as holding machine screws in an appliance cabinet, electric motor housings and such. Usually the internal tooth type are used under the heads of machine screws and the external tooth type under nuts, but I am not sure if their is a specific reasoning behind that practice.
A jamb nut is normally a nut of the same variety, but slightly thinner. Once the main nut is torqued into place an jam nut is tightened against it to hold it firmly in place. This method is used on very high load demands. (next time your at a shopping mall take a look at the base of the parking lot light poles. You will probably see double nuts used to hold the poles on the concrete pedastal pads). This method is also commonly used on critical parts in an automobile engine internal parts such as rod or crank bearings.
Self locking nuts have a fiber or nylon washer built into the nut which has an internal opening slightly smaller than the bolt diameter. As a bolt passes through the nut it cuts into the fiber material which locks it onto the threads of the bolts. These type are very good in high vibration applications such as on your lawnmower.
The knob on your chair probably has a common lock washer.
Your problem in probably resulting from a couple of conditions which should be fairly easy to fix.
You stated that there is a flat washer between the lock washer and the metal base it rests on. No douth the surface under the flat washer is painted. Quite often in an application like that the paint under the washer breaks down, and the minute granules of loose paint will actualy work as a bearing surface allowing the flat washer to move. You could first try roughing up the painted surface and the face of the flat washer with a piece of sandpaper to remove the loose paint and improve the friction on that surface. If that doesn't solve the problem you may try removing the flat washer and allowing the common lock washer to lock between the underside of the knob and the metal frame.
Common lock washers require a considerable torque in order to work most effectively. You may try adjusting the seat to your liking, then use a pair of pliers to torque the bolt down a bit tighter.
