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:

            r_i = inner radius of friction disc
            r_o = outer radius of friction disc

            p_max = maximum allowable pressure on friction surface material

                                      (4)

* Assume: r_i = (0.55- 0.65) r_o

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

T_c = n. μ F_n . r_m

Substitute r_i = 0.6 r_o in the above equation

                                               (5)

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

T_c = b T_{e max} , n, p_o, μ

Take

b=(1.2- 1.6) = 1.6

            n= 2

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

            μ = (0.2- 0.35) = 0.25

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

Substitute in by r_i = 0.6 r_o

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

b) The value of normal force (F_n):

From Eqn. (2);

               (6)

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

Substitute the value of T_c, μ, r_m in Eqn. (6)

- Get the value of (F_n)

c) Check new clutch torque:

         (7)

If T_c (new) ≥ T_c (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:

t_s = 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

            T_c = clutch torque

t_s = 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

            A_shear = z L b

            r_s = (D+d)/4

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

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

            σ_b = 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 (N_max) = 1.2 N_{max Power}

                             (14)

r_o outer  for the driving plate = r_o 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 r_o

{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

           d_b= core bolt diameter

           σ_all = 60 MPa                                                                         

- Get the number and dimension of blots (n_bolt, 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:

t_s = allowable shear strength of rivet material

            σ_b = allowable bearing pressure of rivet material

τ_shear = allowable shear strength of

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

            d = rivet diameter

            F = rivet shear force = (T_c/(d_e/2))/z

            t = thickness of strips group = t_strip 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 (E_steel = 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