Abstract: A method of assembling a tandem pump comprising first and second pumps connected in tandem by an interface. Each pump has a housing and an end cap containing hydraulic porting. The interface connects the end cap of one pump to the housing of the other pump.

Abstract: A tandem pump comprising first and second pumps connected by an interface. An interface for connecting an end cap of a first pump to a housing of a second pump. An interface kit for connecting two pumps in axial alignment to form a tandem pump.

Abstract: A variable frequency control for a hydraulic percussion actuator includes a piston bore and a valve bore. A piston is axially disposed within the piston bore, while a valve is axially disposed within the valve bore. A first piston biasing member biases the piston in a first direction while a second piston biasing member, which overcomes the bias of the first piston biasing member to move the piston in a second direction. A first valve biasing pin biases the valve in a third direction while a second valve biasing pin overcomes the bias of the first valve biasing pin to move the valve in a fourth direction. A hydraulic valve actuator displaces the second valve biasing pin when the piston is extended in the second direction, and a piston actuator activates the second piston biasing pin when the valve is extended in the fourth direction. A valve response member controls the time required for the valve to be displaced in said fourth direction upon actuation of said valve actuating member.