Automotive Clutch Design

 A) The friction (driven) plate dimensions:   a) The value of outer and inner radius (ro) and (ri):   Tc = b Te max                                       (1)   Where:             b = clutch safety factor (1.2-1.6)             Tc = clutch torque capacity             Te max = max. engine torque   Tc = n. Ff. rm          = n. μ Fn . rm                                  (2) Where:             n = number of friction surfaces (number of friction plate x 2) μ = coefficient of friction between friction surfaces Ft = tangential force Fn = normal force rm = the mean radius of friction disc

Clutches are usually designed based on uniform wear (old clutch), then:

(3)

where:

ri = inner radius of friction disc
ro = outer radius of friction disc

pmax = maximum allowable pressure on friction surface material

(4)

* Assume: ri = (0.55- 0.65) ro

Substitute the value of Eqn. (3) and (4) into Eqn. (2) will give:

Tc = n. μ Fn . rm

Substitute ri = 0.6 ro in the above equation

(5)

Substitute in Eqn. (5) by the following values:

Tc = b Te max , n, po, μ

Take

b=(1.2- 1.6) = 1.6

n= 2

pmax = (0.25- 0.6 MPa) = 0.45 MPa (N/mm2)

μ = (0.2- 0.35) = 0.25

- Get the value of (ro)             (Ans.)

Substitute in by ri = 0.6 ro

- Get the value of (ri)             (Ans.)

b) The value of normal force (Fn):

From Eqn. (2);

(6)

Substitute in Eqn. (4) and get the value of (rm)

Substitute the value of Tc, μ, rm in Eqn. (6)

- Get the value of (Fn)

c) Check new clutch torque:

(7)

If Tc (new) ≥ Tc (old)               (O.K.)

 d) Check number of rivets:                                  (8)         Where:             Fr = force applied to rivets             Tc = clutch torque.     i) Check bearing stress     (9)   Where: σbearing = allowable bearing stress  (15 MPa) Abearing = t x dro (where dro and dri are the outer and inner diameter of the rivet) nr = number of rivets

ii) Check shear stress

(10)

Where:

ts = allowable shear strength (10MPa)

for tubular rivets,    for blind rivets.

- Get the value of number of rivets (No. of rivets)

e) Check hub dimensions:

 i) Check shear stress           (11) ii) Check bearing pressure       (12)

iii) Check bending stress

Where

Tc = clutch torque

ts = allowable spline shear strength, (15-30 MPa)

D = Outer diameter of spline

d = Inner diameter of spline

z = number of splines

L = length of spline

b = width of spline

Ashear = z L b

rs = (D+d)/4

σbearing = allowable bearing pressure, (20-30 MPa)

h = spline height = (D-d)/2

σb = allowable bending stress, (20-60 MPa)

- Get the number and dimensions of splines ( z, b, D, L)

{Other parts to be checked: (torsion springs and damping disk)}

B) The pressure (driving) plate dimensions:

a) Check outer diameter for circumferential speed

* Assume maximum rotational speed (Nmax) = 1.2 Nmax Power

(14)

ro outer  for the driving plate = ro outer of driven plate

For cast iron, the centrifugal speed limited by the centrifugal force strength should not exceed 65- 70 m/s

- Check the ro

{Other parts to be checked: (tangential and redial stress)}

C) The clutch cover (housing) dimensions:

a) Check number and dimensions of pins of clutch cover

 Torque transmitted by the cover is Tcover = Tc / 2                                (15)                                        i) Check bearing pressure on the pins            (16)                              ii) Check shear stress                       (17)   - Get the number and dimensions of clutch cover pins (zpin, dpin)

b) Chick number and dimensions of clutch cover bolts

 i) Check tension stress:                                      (18) Where:             Fbolts = Fn + Fstrips + Frelease x lever ratio             σt = allowable tensile strength    * Assume Fbolts  = k Fn , take k= 2.2

(19)

Where:

n = number of bolts

db= core bolt diameter

σall = 60 MPa

- Get the number and dimension of blots (nbolt, bolt size, and length)

{Other parts to be checked: (bolts length, bolts tighten torque)}

c) Check the elastic strips connecting the clutch cover with pressure plate

 i) Check tension stress               (20)                                                 Where:            F = [ T / (de/2) ] / (2 . z . n)            de = pitch circle diameter of the rivets            σt = allowable tensile strength            z = number of elastic strips) group            n = number of elastic plates in the group

ii) Check shear stress (the rivet)

(21)

iii) Check bearing pressure

(22)

iv) Check shear stress (the elastic strip)

(23)

Where:

ts = allowable shear strength of rivet material

σb = allowable bearing pressure of rivet material

τshear = allowable shear strength of

n = number of strips group
z = number of strips in the group

d = rivet diameter

F = rivet shear force = (Tc/(de/2))/z

t = thickness of strips group = tstrip x n

S = length between the rivet center and the edge of the strip

v) Check strain or elongation

(24)

Where:

E = modulus of elasticity (Esteel = 20 x 104 N/mm2)

- Get the dimension of elastic strips (B, L, t)

{Other parts to be checked: (Diaphragm spring)}

D) The slipping of friction clutch and its heat conditions

E) The clutch control drives

Example:

 Symbol Meaning Value Te max Engine maximum torque 150 N m  @ 3700 rpm Pe max Engine maximum power 80 kW @ 5500 rpm n Number of friction surfaces 2 ro Clutch friction plate outer radius ri Clutch friction plate inner radius t Thickness of clutch plates 0.5 mm nr Number of rivets 16 dro Outer diameter of rivet 7 mm dri Inner diameter of rivet 6 mm D Outer splines diameter 20 mm d Inner splines diameter 18 mm b Width of spline 1 mm L Length of spline 25 mm z Number of splines 24 t Thickness of cover plate 4 mm z Number of cover pins 3 Rpins = ro +10 mm Pins pitch circle radius d Diameter of cover pin 6 mm M8 Bolt size n Number of bolts 6 B Strip width 15 mm de Pitch circle diameter of strip rivet d Strip rivet hole 7 mm S Distance between strip rivet hole center and strip edge 6 mm t Strip thickness 0.75 mm L Strip length 64 mm n Number of strips group 3 z Number of strips in the group 2