PCT No. PCT/FI95/00442 Sec. 371 Date Feb. 14, 1997 Sec. 102(e) Date Feb. 14, 1997 PCT Filed Aug. 22, 1995 PCT Pub. No. WO96/07012 PCT Pub. Date Mar. 7, 1996An arrangement for limiting the swing angle of a boom for a rock drilling unit when the boom is being lifted and lowered. In the arrangement, a control member is coupled to the boom, turning therewith. The control member includes a slot which receives a movable dowel pin. The dowel pin is coupled to control a relief valve so that when the boom is being lifted or lowered, and when it tends to turn outside a predetermined widest permissible swing angle, the control member controls the relief valve so that it lets hydraulic fluid into the swing cylinder for turning the boom in a reverse direction.
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