1. Field of the Invention
The invention is concerned with an improvement in the art of translating a carriage relative to outer and inner mast members. Most particularly the invention is concerned with an improvement in a mast assembly as is used on a lift truck.
2. Prior Art
Mast assemblies are well known which include a pair of spaced apart outer mast members, a pair of spaced apart inner mast members between the outer mast members, each of the inner mast members being slidably mounted in a respective one of the outer mast members and a carriage or the like which is mounted on the inner mast members for translation therein. A great number of common lift trucks have such a structure. For example, U.S. Pat. DES. No. 235,056 clearly illustrates a lift truck having such a mast assembly.
A serious problem, has existed with prior art mast assemblies of the type just described. In particular, the strength and stability of such a mast assembly has at times been insufficient for heavy duty applications such as arise in forest logging operations due to the relatively very large moments of force which are exerted upon one or more members thereof, for example upon the carriage thereof. A typical carriage has side thrust rollers which are adequate for light duty applications, however for use in heavy duty applications such as forestry logging operations these are not fully adequate. Further, the spaced apart outer mast members which are generally supporters of the rest of the structure have a tendency to spread apart due to side thrust load causing excessive wear and ultimate premature failure of loaded components. Also, when loads are suddenly taken up, for example on the carriage of such a heavy duty mast assembly, the mast and carriage of the assembly would normally be dragged against one another under relatively high stresses. Very importantly in heavy duty applications undesirable excessive loading of the carriage rollers occurs during impact loading along with cocking of the carriage due to unbalance of the load on the forks of the carriage.