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

 

Description: C:\My Site\subjects\design\clutch_design_files\image001.gif

 

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

 

Description: C:\My Site\subjects\design\clutch_design_files\image002.gif                     (3)

 

where:

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

            pmax = maximum allowable pressure on friction surface material

 

 

Description: C:\My Site\subjects\design\clutch_design_files\image003.gif                                      (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

           

    Description: C:\My Site\subjects\design\clutch_design_files\image004.gif      

Description: C:\My Site\subjects\design\clutch_design_files\image005.gif

 

Substitute ri = 0.6 ro in the above equation

 

Description: C:\My Site\subjects\design\clutch_design_files\image006.gif                                               (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);

 

Description: C:\My Site\subjects\design\clutch_design_files\image007.gif               (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:

 

Description: C:\My Site\subjects\design\clutch_design_files\image008.gif         (7)

 

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

 

 

d) Check number of rivets:

                

Description: C:\My Site\subjects\design\clutch_design_files\image010.gif                (8)      

 

Where:

            Fr = force applied to rivets

            Tc = clutch torque.

 

 

i) Check bearing stress

 

Description: C:\My Site\subjects\design\clutch_design_files\image011.gif  (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

 

Description: C:\My Site\subjects\design\clutch_design_files\image012.gif                        (10) 

 

Where:

            ts = allowable shear strength (10MPa)

            Description: C:\My Site\subjects\design\clutch_design_files\image013.gif    for tubular rivets,   Description: C:\My Site\subjects\design\clutch_design_files\image014.gif for blind rivets.

 

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

 

e) Check hub dimensions:

 

i) Check shear stress

 

Description: C:\My Site\subjects\design\clutch_design_files\image016.gif Description: C:\My Site\subjects\design\clutch_design_files\image017.gif       (11)

ii) Check bearing pressure

 

Description: C:\My Site\subjects\design\clutch_design_files\image018.gif    (12)

 

Description: C:\My Site\subjects\design\clutch_design_files\image015.gif

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

 

Description: C:\My Site\subjects\design\clutch_design_files\image020.gif                             (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

 

Description: C:\My Site\subjects\design\clutch_design_files\image022.gif         (16)                           

 

ii) Check shear stress

 

Description: C:\My Site\subjects\design\clutch_design_files\image023.gif                    (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:

 

Description: C:\My Site\subjects\design\clutch_design_files\image025.gif                                   (18)

Where:

            Fbolts = Fn + Fstrips + Frelease x lever ratio

            σt = allowable tensile strength 

 

* Assume Fbolts  = k Fn , take k= 2.2

 

Description: C:\My Site\subjects\design\clutch_design_files\image024.gif

 

Description: C:\My Site\subjects\design\clutch_design_files\image026.gif                                 (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

 

Description: C:\My Site\subjects\design\clutch_design_files\image028.gif            (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

 

Description: C:\My Site\subjects\design\clutch_design_files\image027.gif

 

ii) Check shear stress (the rivet)

 

Description: C:\My Site\subjects\design\clutch_design_files\image029.gif                       (21)

 

iii) Check bearing pressure

 

Description: C:\My Site\subjects\design\clutch_design_files\image030.gif                      (22)

 

iv) Check shear stress (the elastic strip)

 

Description: C:\My Site\subjects\design\clutch_design_files\image031.gif                  (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

 

Description: C:\My Site\subjects\design\clutch_design_files\image032.gif                                     (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