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MAKE-UP HINGE STRAPS can be made of any flat resilient flexible material (hard rolled type 302 stainless steel has proved ideal in many instances). Strap THICKNESS is dependent on the rolling radius of the Hinge Supports together with the required strength and durability of the Hinge. Strap WIDTH is dependent on the required SHEAR loading. Straps are loaded 'sprung' flat so that, when pre-tensioned, the 'spring' is nullified by the opposing straps and their opposing curvatures. HINGE SUPPORTS can be made of any resilient material (more often than not tubular in shape), their only critical area being the ROLLING RADIUS for a particular hinge's desired angle of rotation. HINGE PLATES can be made of any resilient material. Length and thickness is dependant on the desired form of fixing and the required overall strength and durability (flat mild steel or shallow U-section stainless steel are ideal for SPOT WELDING and both allow for some flexibility and pre-tensioning when FIXING the Hinge Plates together). Due to the friction between the Hinge Straps and the Hinge Supports when under tension, only moderate anchoring of the plates to the Supports is required to prevent misalignment. Where large shear loads are involved, the individual Hinge Plates are screwed or bolted directly to the Hinge Supports (with or without spring tensioners). STRENGTH TENSILE STRENGTH (plane 1) is dependant on the combined effect of the tensile strength of Hinge Strap, the position of rotation in relation to the Hinge Plates, and the strength and form of the Plate and Strap fixings. (‘safety’ rule of thumb for hard rolled type 302 stainless steel: approx 250kg per 25mm total hinge strap width per 5thou-inch hinge strap thickness) COMPRESSIVE STRENGTH (plane 1) is as strong as the weaker of the two compressive strengths of the material of the Hinge Straps and the Hinge Supports. SHEAR STRENGTH (plane 2) is dependant on the combined effect of the Hinge Strap thickness, the total Strap width and the correct tensioning of the Straps. (‘safety’ rule of thumb for hard rolled type 302 stainless steel: approx 200kg per 25mm total hinge strap width per 5thou-inch hinge strap thickness) RIGIDITY IN TENSION (plane 1), the separation of the Hinge Supports increases exponentially up to approximately 10% of breaking strain, thereafter the increase in separation is inverse to the increase in tension until breaking strain is reached. This unusual behaviour is due to a combination of the initial tensioning of the Hinge Straps and the changing angle of crossover of the Straps. IN COMPRESSION (plane 1), with Hinge Straps properly tensioned, movement is minimal and is generally measured in thousandths of an inch. IN SHEAR (plane 2), with Hinge Straps properly tensioned, no movement is detected up to approximately 15% of shear load, thereafter sudden movements occur that misalign the Hinge Straps and greatly reduce HINGE LIFE. LIFE (measured as the number of reversible turns) Hinge life is dependent on the ratio of Hinge Strap thickness to Hinge Support diameter and on the correct tensioning of the Straps. For indefinite life (2 million plus reversals), the Hinge Straps must at all times be kept within their elastic limit and all rolling surfaces on the Hinge Supports must be as near as possible perfectly circular. (hard rolled type 302 stainless steel indefinite life rule of thumb: rolling radius must exceed 180 times strap thickness) However, if angular movement is kept within approximately 10 degrees (equal to 5 degrees on the rolling radii), then the Hinge Straps are able to be ‘x2’ in thickness. At this angular movement, the Hinge Straps do not experience a reverse deflection from one Hinge Support to the other and therefore the Straps remain as much within the their elastic limit as the double deflection ‘x1’ thickness Straps. For reliable indefinite life, two other factors are important. Firstly, foreign particles must be prevented from entering between the Hinge Straps and the Hinge Supports, as they can damage both the Straps and the Supports during rotation. Secondly, moderate and even tensioning of the Hinge Straps is essential, so much so that life expectancy can be reduced to as little as 100,000 reversals if the Straps are either too loose (at almost zero tension) or in excess of approximately 20% of breaking strain. In the first instance, the Hinge Straps move on the Hinge Supports and the Straps distort and, in the second, the excess tension can compromise the elastic limit of the Strap material during rolling. FRICTION When defined as the ‘rubbing together of two bodies’ & ‘coefficient of friction’ (Oxford English Dictionary), there is effectively no friction. Tests with double-beam weighing scales (indefinite-life SRHs replacing the knife edges) shows no noticeable variation in movement with the smallest detectable mass applied (0.5gm) at the scales' maximum practical load (100Kg). Furthermore, multi-million turn tests on raw aluminium Hinge Supports produce no visible additional marking of the raw aluminium after the first few hundred turns. After an initial ‘bedding in’ the Hinge Straps appear to return to their exact crystalline position on each reversal, leaving only a molecular interaction with which to contend. STICTION (As used in the aeronautics industry) Tests indicate that with indefinite life SRHs, ‘stiction’ is extremely low. However, without extensive tests and further calculations, this loss of energy is difficult to separate from total energy loss. TOTAL ENERGY LOSS As stated above, 0.5gm will visibly move the weighing scales' 6 indefinite-life Hinge pairs, each of which has a load capability in excess of 250kg. Within the practical limits of the tests, this 0.5gm appears to be constant, which indicates a virtual nil ‘coefficient’ of energy loss and an ‘actual’ energy loss at maximum practical loading (150kg) approaching 1 part in 1/4 million. These results did not noticeably vary when loading of the weighing scale Hinges was changed from tension to compression. CHARACTERISTICS OF ROTATION The SRH and its attachments rotate in an epicycloid motion, as against a circular motion for conventional hinges. However, for all practical purposes, the arc described by any point on an attachment to an SRH for 90 degrees of rotation either side of the central position will have a 'virtual' fixed centre of rotation. The position of this centre of rotation varies dependant on the distance from the Hinge Support, and falls on the centre line of the two Supports within the fixed Support and between its own centre point and its rolling radius. This principle is useful for approximating to conventional rotation when no more than 180 degrees of total movement is required (there is no mathematical formula established for this as yet). |
