A friction welder has a reciprocator (1) with two chambers (7 and 8) each containing hydraulic fluid and separated by a dividing wall (6). A piston (14) on a driven rod (13) is slidable in a hole in the dividing wall and supports a carrier (28) to which a first component (36) is secured. Tow additional rods (9 and 10) extend through the chambers. Enlarged portions (11) thereof extend through holes in end walls (4 and 5) of the chambers. The additional rods are caused to reciprocate with fixed amplitude by Scotch cranks (23 to 26, and 31) driven by a motor (30). The relative phases of the Scotch cranks can be varied by axial movement of a sleeve (35) which engages splined shafts of the cranks, one of sets of splines (35) being helical and the other straight. When the additional rods reciprocate in unison they cause local variations in the interior space of the chambers which cause reciprocation of the piston. The amplitude of reciprocation of the piston is less than that of the additional rods but the force exerted is greater. The first component is held in frictional contact with a stationery second component (37) so that frictional heat is generated between them. When the cranks are 180 DEG out of phase the piston ceases to move and the components become welded together. Reciprocation of the piston and associated parts is balanced by a balancer (2) somewhat similar to the reciprocator, while reciprocation of the additional rods and their extensions (16 and 17) in the balancer are balanced by pivoted counterweight (46).
