Conventional heavy-duty lead screw assemblies consist of a hard steel screw with an acme thread and a metal, such as bronze, nut with a matching thread. An acme thread usually has a built-in clearance so that there can be relative movement between the screw and the nut when the screw goes from a "pushing" to a "pulling" mode or from a loaded to an unloaded mode. Lubricant tends to collect in the clearance spaces and is partially extruded from one clearance into another as the load direction changes so that the screw and nut are lubricated. While this combination of materials is ordinarily satisfactory, it was found to fail under certain combinations of conditions.
In a situation where a lead screw assembly is used to position a base carrying a variable sheave motor for the fan of a central air conditioning system, the load is unidirectional and continuous since spring tension is employed to maintain the proper tension on the fan belt. The lead screw acts against the spring tension of the variable sheave to move the motor base in one direction and retards the return movement of the spring in the opposite direction so that the lead screw is always opposing the spring tension of the variable sheave and the same sides of the screw threads are always in engagement when the device is in operation. Depending upon the axial length of the nut and the stroke of the lead screw, lubrication of the contacting portion of the screw thread may be possible as it is withdrawn from the nut, but it would have to be done more frequently than normal maintenance practice would indicate.
The problem of lubrication in the case of continuous unidirectional loading is compounded by the location of the variable sheave motor, and therefore the lead screw assembly, in proximity to the evaporator coil of the central air conditioning system which results in a high humidity environment. The combination of poor lubrication and high humidity produces corrosion and accelerated wear of the lead screw assembly. Corroded metal in the form of rust is broken off of the threads and is trapped by the lubricant and acts as an abrasive causing accelerated wear and failure of the parts. This situation exists for various standard combinations of parts such as steel lead screws in combination with bronze, brass, cast iron and nylon nuts, all of which were subject to failure in this manner. Stainless steel, which may appear to offer a solution, is subject to galling, the condition that makes it unsuitable for use as a bearing, and it too fails from excessive wear. Plastic is not suitable since it lacks the necessary rigidity for use as a lead screw in the length required.