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Mechanical Engineering-Tribology In Machine Design Episode 6

Tham khảo tài liệu 'mechanical engineering-tribology in machine design episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 112 Tribology in machine design radius. Let Tj and r2 denote the maximum and minimum radii of action of the contact surfaces R the total axial force exerted by the clutch springs and Ha H l the number of pairs of active surfaces. Case A uniform pressure intensity p R n r2-r22 p 4.36 driving couple for each pair of active surfaces n p rf r2 1 2 . . 2ịrị rỊ total driving couple Kna ---44 3 rf-rp 4.37 Case B uniform wear pr c If p2 is the greatest intensity of pressure on the friction surfaces at radius r2 then P2T2 c R 2nC r r2 or R 27tp2r2 rỉ -r2 4.38 Driving couple for each pair of active surfaces fRj rl r2 total driving couple fRnỉf rì r2 4.39 Comparingeqns 4.37 and 4.39 it is seen that the tangential driving force F fR can be reduced to a mean radius rm namely for uniform pressure rm ị ----ị 4.40 ri r2. for uniform wear rm rj r2 4.41 Numerical example A machine is driven from a constant speed shaft rotating at 300 r.p.m. by means of a friction clutch. The moment of inertia of the rotating parts of the machine is 4.6 kgm2. The clutch is of the disc type both sides of the disc being effective in producing driving friction. The external and internal diameters of the discs are respectively 0.2 and 0.13 m. The axial pressure applied to the disc is 0.07 MPa. Assume that this pressure is uniformly distributed and that the coefficient of friction is 0.25. If when the machine is at rest the clutch is suddenly engaged what length of time will be required for the machine to attain its full speed. Friction lubrication and wear in lower kinematic pairs 1Í 3 Solution For uniform pressure p 0.07 MPa the total axial force is R n r2-rị p R 3.14 0.12 - 0.0652 70 000 1270 N. Effective radius 7r2-r22 70.13 0.0653 rm j -2 -ị ị 73 0.084 m. r2-r22 30.12 0.0652 Number of pairs of active surfaces Ha 2 then friction couple fRnarm 0.25 X 1270 X 2 X 0.084 53.34 Nm. Assuming uniform acceleration during the time required to reach full speed from rest __ couple angular .