Patent GB-771480-A (GB)
Title: Improvements in measuring instruments
IPC: G01P3/495
Filed: 1954-03-02
Abstract: 771,480. Indicating-apparatus; speedometers. GENERAL ELECTRIC CO. March 2, 1954 [March 3, 1953], No. 6120/54. Classes 106 (3) and 106 (4). Indicating apparatus with a rotatable shaft 11 moved by a torque proportional to a quantity to be measured with different torquedeflection characteristics over adjacent ranges comprises separate springs 20, 19 for restraining motion over the two ranges and a second shaft 16 concentric with but rotatable independently of the first shaft, the first spring 20 restraining motion of the first shaft 11 relative to the second shaft 16 and the second spring 19 restraining motion of the second shaft relative to a fixed support, arranged so that after a predetermined movement the first shaft is operatively connected to the second shaft. The invention is shown applied to a tachometer wherein a small pointer 2 indicates up to 50 per cent engine speed against an open arcuate scale extending over 225 degrees while a large pointer 3 moving as one with the small pointer 2 indicates from 50-100 per cent engine speeds against a relatively contracted scale extending over 125 degrees. Indications are derived responsive to the speed of the output shaft 6 of a synchronous motor 7 which speed is proportional to that of an engine. The shaft 6 drives a magnet assembly 8 comprising oppositely disposed sets of permanent magnets 9 which urge an eddy-current drag disc 10 to rotate. The disc 10 is attached to a main pointer shaft 11 which carries the small pointer 2 and is supported in a fixed jewel bearing 12 and in jewel bearings 14, 15 within an outer pointer shaft 16 carrying the large pointer 3 and itself supported in a fixed jewel bearing 17. Spiral springs 19, 20 exert torques on the pointer shafts 16, 11, in the opposite direction to that in which the disc 10 tends to deflect shaft 11. The outer pointer shaft 16 carries a stop arm 21 engaging a stop tab 22 on an adjustable bracket 23 which has another tab 26 to which is attached the outer end of the spring 19 whose inner end is attached to the shaft 16. A suppressed torque is introduced by winding the spring 19 before engaging the stop arm 21 behind the stop tab 22, a fine adjustment of the spring being obtained by adjusting the bracket 23 and of the stop by bending the tab 22. Upscale motion of the pointer 3 occurs only after main shaft 11 has rotated beyond an angular position at which an arm 27 attached to the main shaft 11 engages and drives a contact arm 28 attached to the outer pointer shaft 16, adjustment of this angular position being obtained by bending either or both of the arms 27, 28. Spiral spring 20 is attached at its inner end to shaft 11 and at its outer end to shaft 16 and alone exerts a torque on shaft 11 until the arms 27, 28 engage, thereafter the pointers 2, 3 are superimposed and only spring 19 exerts a torque If dual pointers are not required the pointer actuated by the outer shaft may be omitted. Instead of pointers, electrical pick-off devices may be actuated by relative movements between elements of the mechanism to provide electrical signals characterizing these movements. The invention may be applied to ammeters and voltmeters.