Clamping device, especially for building materials, has width automatically adjusted using mechanical rotary switch device connected to hydraulic distributor valve

The device has at least two movable arms (O, P) which are pivotally connected to a coupling piece securable to a movable lifting device in a crane. The ends of the arms are provided with U-shaped clamping jaws (S, T) which can engage with the product to be gripped by the device. At least one rubber grip is located in the jaw and extends over at least part of the jaw length. The width of the clamping device is altered using a mechanical rotary switch device which uses a hydraulic distributor valve to select the hydraulic cylinders (X, Y) to be operated. The valve is operated by a cam which is slidably mounted in a slot and controls the roller mechanism of the valve.