Abstract: A tensioning frame for stretching and keeping stretched upon it a canvas sheet in a plane configuration and with uniform tension, some or all of the outer edge of the canvas sheet being capable of sliding motion over the outer edge of the rigid structure covered by it and attached to it by means of springs. The force of each spring may be adjusted in situ to the tension desired in the canvas sheet. The tension and plane configuration of the canvas sheet is thus maintained in spite of the effects of temperature, humidity, or weight. Sliding of the contacting surfaces is facilitated by layers of a low-friction substance.

Abstract: A safety valve for pneumatic tools, comprising first and second valve bodies which are movable relative to each other and which carry cooperating seat surfaces. The first valve body is of tubular shape and is displaceable between a closed and an opened position by means of a lever.The second valve body is pressure balanced and biased by a spring to maintain sealing contact with the first valve body as the latter is displaced. A lever operated arresting means is arranged to lock the second valve body against movement and thereby interrupt the sealing contact between the first and second valve bodies as the first valve body is displaced.