From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article discussing mechanical torque amplifiers and the Bethlehem Lashlock for backlash elimination.
| Identifier | ExFiles\Box 154\3\ scan0062 | |
| Date | 26th November 1923 | |
| PAGE 1: [Handwritten note] A.A. pins operated by hand wheel. B.B. Pins receiving power & torque. C. Lashlock - see p. 6. C. Carpenter 26.11.23 Fig. 5.—Automobile amplifier with housing removed, showing friction bands, control arms and “lashlock” for eliminating backlash. The assembled unit is shown in Fig. 4 shaft of the second amplifier, and so on, the work shaft of the final amplifier performing the required work. Each amplifier in the series will multiply the torque it receives by 40. Thus, with two amplifiers the ratio of amplification would be 1 to 1,600; with three the ratio would be 1 to 64,000, etc. In actual design amplifiers are built in “stages,” the first friction band contracting on the hub of a drum and the second expanding within the outer flange of the same drum; while the third band and the fourth contact similarly with a second drum. The bands are hooked together through a system of simple linkage. Two types of amplifiers are shown in Figs. 4, 5 and 6. In amplifiers of certain types a spring may be used to perform the drive in one direction, or (in which case the torque on the work arm is limited, in that direction, to the strength of the spring) or by actuating the control of the final stage of a multi-stage amplifier (in which case the torque in both directions is unlimited in amount, to all intents and purposes). Possible uses of the torque amplifier include such a range as automobile and ship steering, gun turning, switch and signal control, control of steam systems, running of tower clocks, operating valves, opening or revolving doors, metering devices, governors, various machine tool mechanisms, and regulation of roll speeds in steel, paper and textile mills. Fig. 6.—Service amplifier, a light duty type, adaptable to computing machines, distant control of rheostats and similar situations [ 4 ] PAGE 2: The Bethlehem Lashlock An automatic backlash eliminator that is suitable to many mechanical movements THE principle of the Bethlehem lashlock (Nieman patents) was discovered by an engineer of the Bethlehem Steel Company during the construction of a heavy-duty machine (the torque amplifier, described in the last issue) in which it was particularly desirable that there should be no backlash. Development of the original idea has shown that it can be successfully applied to a wide range of machinery, from large mechanisms involving great forces to small and delicate instruments. The lashlock operates to eliminate backlash in gearwheels, feedscrews, end-thrusts, bearings—practically wherever play occurs between two members. It also compensates for wear, and automatically relieves strain caused either by tight setting or expansion due to heat. Action of the lashlock is positive, without slip or yielding under load, and in no way lessens the power which can be transmitted or weakens the members to which it is applied. Entirely automatic, it never requires hand adjustment, and needs only the minimum of lubrication. The basic principle of the lashlock will be seen from Fig. 1. A wedge-shaped member, A, slides on a stem, B, and is urged downward by the light spring C. Two rollers, D and E, are disposed so that they press against the wedge, as shown. It is obvious that if both rollers press simultaneously against the wedge it will be forced upwards, readily overcoming the pressure of the light spring. What may not be so obvious at first glance is no matter how great the force applied to the single roller, as the proportion between the friction on the stem and the upward force imparted to the wedge remains the same. When both rollers are pressed simultaneously, however, the pressures are balanced and no friction results between wedge and stem; thus all the force applied to the two rollers is brought to bear on moving the wedge upward. From this examination it is seen that the lashlock depends upon two things: a frictional locking when only one roller is pressed; and a complete release when both rollers are pressed simultaneously. Fig. 1—Wedge type of lashlock. Fig. 2—Lashlock applied to split gears Fig. 3—Wedge lashlock applied to leadscrew that, with the slope of the wedge approximately as shown (5 deg. to 10 deg. on each side of the axis), if only one roller is pressed against the wedge the friction between the wedge and stem will be greater than the upward force imparted to the wedge, and the wedge will bind upon the stem. All the force applied to the single roller will thus be imparted to the member which supports the stem, forcing it in the direction in which the roller presses. This binding or locking effect will occur Application of the above working principle to mechanical movements can be accomplished in a variety of ways, provided the essential elements are always included. Take, for example, a pair of gearwheels, a mechanical combination notorious for backlash — in which, as a matter of fact, a certain amount of backlash has always had to be allowed for in the design and setting. Several methods have been used to mitigate the backlash in gears, perhaps the most widely known being the “split” gear, where two gears are mounted side by side on the same shaft, one gear keyed to the shaft and the other free to turn. Both of this split pair mesh with a single solid gear mounted and keyed on a second shaft. The split pair is provided with a spring which tends to rotate the free gear, causing the teeth of the split gears to pinch the teeth of the single-gear element with which they mesh. With a split gear one of the split pair performs the drive in one direction, the other in the opposite direction, and it will be seen that the torque in one direction is positively transmitted, while in the other direction it depends upon the strength of the spring. This fact limits the use of split gears to [ 5 ] | ||