Clutches and Brakes Des Mach Elem Mech. Eng. Department Chulalongkorn University

Transcription

1 Clutches and Bakes 0330 Des Mach Elem Mech. Eng. Depatment Chulalongkon Univesity

2 ntoduction ( Clutch and bake ae the machine elements used in tansmission and otation contol. Hee only clutch and bake using the concept o iction between suaces ae ocused. Engage disk with disk, Both o disks ae bought to the same speed by the eect o iction. (Clutch Speed o disk is deceased. (bake Clutch Bake 0 < 0

3 ntoduction ( o design clutch o bake, the ollowings must be consideed. he actuating oce: he oce used to pess clutch o bake to engage. he toque tansmitted 3. he enegy loss om the slippage between two suace 4. he tempeatue ise due to the enegy loss ypes o clutch and bake. Rim types with intenal expanding shoes. Rim types with extenal contacting shoes 3. Band types 4. Disk o axial types 5. Cone types 6. Miscellaneous types

4 ypes o clutch and bake ( Rim types with intenal expanding shoes Rim types with extenal contacting shoes Band types

5 ypes o clutch and bake ( Disk o axial types Clutch in automobile Disk bake & bake pad Cone types

6 ntenal Expanding Rim type ( Pessue distibution along the intenal shoe p p a θ sinθ sin a a : Maximum pessue long shoe, θ > 90, Max, pessue is occuing at 90 shot shoe, θ < 90, Max, pessue is occuing at the end o the shoe θ p a depend on mateial and elate closely to the iction coeicient x-axis: cente to the hinge-pin y-axis: nomal to x-axis and point to bake shoe Nomal oce dn pb dθ pab sinθ dθ sinθ b is the ace width o the iction mateial a

7 ntenal Expanding Rim type ( Moment about point A (Hinge pin M dn ( a cosθ θ pab sinθ ( a cosθ sinθ a θ dθ M N dn ( asinθ paba sinθ a θ θ sin θ dθ he actuating oce, F [ M 0] A F M N c M M N M, sel-locking is obtained, and no actuating oce is equied to oce a bake pad to engage with a dum. his eect is also called the sel-enegizing eect.

8 ntenal Expanding Rim type (3 he toque applied to the dum by the bake shoe he sum o toque om the ictional oce about the cente o the dum dn pab sinθ a θ θ sinθ dθ p a b Hinge-pin eactions [ F 0] x R [ F 0] y R (cosθ cosθ sinθ a dn cosθ dn sinθ x F x dn sinθ + dn cosθ y F y

9 ntenal Expanding Rim type (4 the otation o the dum is evesed he actuating oce, F [ M 0] A F M N + c M Sel-enegizing eect is lost in this case. Moe actuating oce is equied to geneate the same baking toque he calculation o the hinge-pin eaction is the same. Only the diection o iction is dieent.

10 Example he bake shown in the igue is 300 mm in diamete and is actuated by a mechanism that exets the same oce F on each shoe. he shoes ae identical and have a ace width o 3 mm. he lining is a molded asbestos having a coeicient o iction o 0.3 and a pessue limitation o 000 kpa. Estimate the maximum (a Actuating oce F (b Baking capacity (c Hinge-pin eactions [Ex.6- Shigley s Mechanical Engineeing Design 9 th Edition. Richad G. Budynas and J. Keith Nisbett]

11 Fictional-Contact Axial Clutches. Do not have the centiugal eect. Lage ictional aea and can be installed in a small place 3. Eective heat-dissipation suaces 4. Slippage is possible, hence the impact oce duing engagement is low. Engagement at high speed is pobable. Wea o the clutch disk. Fo a new clutch, clutch disks ae igid. he geatest amount o wea will occu in the oute aeas. (uniom pessue but the oute aeas ae moe slippage 3. Ate a cetain amount o wea has taken place, the pessue distibution is not uniom, but the wea is uniom. Dive Diven Actuato Assumption used in calculation. Uniom wea. Uniom pessue

12 Uniom Wea Fo uniom wea p (constant p : Contact pessue p a : Max. contact pessue p pa i p a d : adius : angula velocity he actuating oce, F F D / D / π p d π pad d d / oque, D / d / D / π p d π pad d d / d / π pad ( D d π p 8 a d ( D d F ( D + d 4

13 Uniom Pessue Fo p p a constant otal actuating oce, F pa F π ( D d 4 oque, D / π pa d / d π p a ( D 3 d 3 F D 3 d 3 3 D d

14 Uniom Wea : Uniom Pessue. ansmitted toque calculated om uniom wea assumption is always lowe than toque calculated om uniom pessue assumption.. Fo d/d > 0.6, both method give almost the same esult. 3. Uniom wea assumption is equently used, because it is sae than the uniom pessue assumption. 4. Many o disks ae pobable used to incease the tansmitted toque.

15 Disk Bakes ( Calculation can be done by the same method as clutch calculation θ o F p d dθ θ θ θ i ( o i p d θ o θ i p d dθ ( θ θ o i p d Equivalent adius, e F e Position to cente o oce, F M x e F o i o i p d p d M x M F F x θ θ o i (cosθ θ θ p( sinθ d dθ cosθ e

16 Disk Bakes ( Uniom wea By uniom wea assumption θ o F p d dθ θ θ θ i θ o θ i p F pa i p Actuating oce, F p a d ( ( θ θ p o i p d dθ ( θ θ ( d a i o i ansmitted toque, Equivalent adius, e e ( θ θ o i p d ( pai o i Position to cente o oce cosθ cosθ θ θ o + i o i

17 Dum Bakes : Disk Bakes Dum bakes Can be designed o selenegization. he baking eot can be educed. Disk bakes Has no sel-enegization. Shoe and lining Piston Bake luid Slight change in coe. o iction (om tempeatue will cause a lage change in the pedal oce equied o baking. Moe oce is equied to obtain the same baking toque Not so susceptible to change in the coeicient o iction Floating calipe bake design help to compensate o wea and ensue a aily constant pessue ove the aea o the iction pads Baking disk

18 Example wo annula pads, i in, o 5.50 in, subtend an angle o 08, have a coeicient o iction o 0.37, and ae actuated by a pai o hydaulic cylindes.5 in in diamete. he toque equiement is 3,000 lb in. Fo uniom wea (a Find the lagest nomal pessue p a. (b Estimate the actuating oce F. (c Find the equivalent adius e and oce location. (d Estimate the equied hydaulic pessue [Ex.6-3 Shigley s Mechanical Engineeing Design 9 th Edition. Richad G. Budynas and J. Keith Nisbett]

19 Enegy Consideations ( Capability o a clutch o bake is limited by actos,. he chaacteistics o mateial (coe. o iction, sae nomal pessue. Ability to dissipate heat t 0 t t Opeation is completed ansmitted toque empeatue-ise poblem Assumption: wo shats ae igid and that the clutch toque is constant θ θ ansmitted toque, depends on shape, coe. o iction, max. pessue

20 Enegy Consideations ( Relative velocity t t t θ θ θ ime equied o the entie opeation t 0 θ θ θ ( ( t + θ θ θ + t θ + t he ate o enegy-dissipation Sliding loss + t u θ otal enegy-dissipation + t t dt t dt u E 0 0 ( ( E +

21 Enegy Consideations (3 otal enegy dissipated can also be calculated by using enegy method t 0 t t Opeation is completed 0 E + E + ( ( 0 E E E + θ + t θ + t + t + t + t ( ( E + otal enegy-dissipation

22 Enegy Consideations (Bake Fo bake, only dums o disks otate, bake pads ae stationay pats 0 t 0 0 t t 0 Opeation is completed E 0 E otal enegy-dissipation E E 0 E E (

23 empeatue Rise otal enegy dissipated is changed to be themal enegy and lead to tempeatue ise. E mc p m : mass o clutch o bake pats, kg C p : speciic heat capacity 500 J/(kg- C o steel o cast ion : tempeatue ise, C Heat om enegy dissipated is tansmitted to envionment, hence the tempeatue is decease. : tempeatue at time t : initial tempeatue : envionment tempeatue h CR : oveall coe. o heat tans. h CR A exp t WC p A : lateal suace aea W : mass o the object C p : speciic heat capacity he eect o clutching o baking opeation on tempeatue. he tempeatue ise may be dieent o each opeation.

24 Fiction Mateials ( Aea o Fiction Mateial Requied o a Given Aveage Baking Powe Some Popeties o Bake Linings

25 Fiction Mateials ( Fiction Mateials o Clutches

26 Fiction Mateials (3 Chaacteistics o Fiction Mateials o Bakes and Clutches