Patent GB-690223-A (GB)
Title: Improvements in or relating to pit props
IPC: E21D15/38
Filed: 1949-10-13
Abstract: 690,223. Pit-props. EISENWERK WANHEIM GES. Oct. 13, 1949 [Dec. 18, 1948], No. 26353/49. Class 20(ii) In a telescopic pit-prop in which the inner prop member is held frictionally in a locking device on the outer member, a band d Fig. 1, or bars p, Fig. 2, extends along the length of and is secured to the inner prop member c or o so as to be positively moved therewith upon penetration thereof into the outer member and to be subjected in the longitudinal direction, by the frictional forces in the locking device, to tensile stresses only. As shown in Fig. 1, a U-shaped band d, bent round the base of the slightly tapered inner member c, is spaced therefrom by a block f at the base of the member c and its two loose upper ends are linked to each other and to the inner member c by a bolt e. Flat bars, g, h are disposed in the lock b extending through the spaces between the inner member c and the two limbs of the band d and are of vertical height slightly less than that of the apertures in the lock b. An operating wedge k acts upon a filling piece i which abuts on the outside of the adjacent limb of the band d. On setting of the prop it is arranged, e.g. by spring means, that the bottom edges of the bars g, h are spaced from the lower edges of the lock apertures. On movement of the roof, the inner member c together with the band d begins to pentrate into the outer member and carries with it the bars g, h, until they abut on the lower edges of the lock apertures. In the modification shown in Fig. 2, the I-sectioned inner prop member o is slightly tapered along its web and straddling the web are two U-shaped bars p of tapered'section having bottom transverse pieces q on which the base of the inner member o bears. The wedge-shaped spaces so formed are engaged by wedge shaped elements s, t which are freely movable longitudinally in the lock n and are adapted to abut on brackets u welded to the outer member m. An operating wedge v causes the elements s, t to move inwards and thus frictionally clamp the inner prop member. Penetration of the inner prop member causes the elements s, t to be forced more deeply into the spaces between the bars p and the flanges of the inner member o and thus increases the frictional resistance. To increase the frictional resistance, the band d or bars p, or the parts of the locks including the elements g, h, i, s, t may be made of or lined with light metal, the' principal material of the prop being steel. The lock elements g, h, s, t may be made wedgeshaped in the vertical direction or one of the elements g, h, or s, t may be tapered in the reverse direction, a yieldable element being inserted between the elements so tapered and the edge of the lock so that at a predetermined load, it is crushed or sheared. Since parts d or p are subjected to tensile stresses only, those parts may be formed of high tensile steel of light construction. Instead of or in addition to tapering the inner prop member itself, the band d or bars p may be tapered longitudinally and in the latter case the elements s, t would be shaped accordingly.