[ 223 ] 



XVII. 



IMPEOVEMENTS IN EQUATOEIAL TELESCOPE MOUNTINGS. 



By sir HOWARD GRUBB, F.R.S., 



Vice-President, Royal Dublin Society. 



(Plates XVII.-XIX.) 



[Read Notember 28, 1911. Published March 26, 1912.] 



Anti-Friction Arrangements for Polar and Declination Axes or 

 Heavy Equatorial Instruments. 



Polar Axis. — The great improvements in the manufacture of ball and 

 cylinder bearings of late years, owing to the general adoption of this form 

 of anti-friction apparatus for so many purposes, has naturally led to its 

 adaptation to large equatorial mountings, and in the most up-to-date 

 instruments we have adopted a necklace or ring of hardened steel rollers for 

 the large upper bearing of polar axis where nearly all the friction takes place, 

 and a ring of balls running between hardened steel plates for tlie end thrust. 

 The side-thrust on lower bearing of polar axis is so slight (the instrument 

 being nearly balanced on the large upper bearing), and the axis itself is so 

 small here in diameter, that there is no necessity to apply any apparatus to 

 relieve the lateral friction on that pivot. 



It should be premised that, as the two essential features wliich require to 

 be kept in view are those of precision of motion and freedom of motion, the 

 general principle adopted is to allow the axes to revolve on or in Y bearings, 

 but to relieve these bearings of about 19/20ths of the weight by some efficient 

 anti-friction arrangement, leaving only the remaining l/20th to ensure the 

 necessary precision of movement. 



Attempts have been made to cany tlie axes of equatorial instruments 

 entirely on friction wheels or rollers ; but while this ensures the condition of 

 freedom of motion, it does not provide for the necessary precision, whereas, by 

 adopting the combined system of Y bearings coupled with the anti-friction 

 rollers, both conditions are fulfilled. 



The single roller under the large upper bearing of polar axis used in the 

 older instruments is therefore now replaced by a necklace of live rollers of 

 hardened steel rolling on the inside of a ring of hard metal as shown in figs. 

 1 and 2, Plate XVII. (See p. 228.) 



SCIENT. PBGC. E.D.S., VOL. XHI., NO. XVII, 2 M 



