Abstract: This paper analyzed the fundamental parameters of gear reducer series type ZQ they are in wide use for cranes, some problems have been dissected in the parameters, Meanwhile some new parameters have been studied for 5-50 tf overhead travelling cranes. The new parameters will be of benefit to rational arrangement of transmitting power, raise of load-carrying capacity of reducer, decrease of the size and weight of reducer, and also improvement of lubrication condition of gear,etc.