Abstract: This invention relates to operating mechanisms for hydraulic control systems and it is an object of the invention to provide an operating mechanism which is self-locking. The mechanism includes a bar which is slidable in a channel, one face of the bar being cut away to provide two inclined surfaces. Rollers are retained by springs in the spaces between the inclined surfaces and one side of the channel. A plate is also located between the bar and the said side of the channel and a control member serves to displace this plate along the channel. The arrangement is such that, when the plate is displaced in either direction, it pushes a respective one of the rollers towards one end of the bar, thereby allowing it to roll down the inclined surface. On the other hand, if a force is applied to the bar in either direction, a respective one of the rollers will move up the inclined surface and wedge against the side of the channel, thereby locking the bar in position.