1. FEAD Basics F = Q/R TT = F + TS TT Q=Torque Q=FxR Ts Where:
TT = Tight side tension
Ts = Slack side tension
F = Force applied at pulley pitch radius
R = Pulley Pitch radius

2. Accessories downstream from the crankshaft will have higher hubloadsQ2
Tsys + T1+T2
Tsys + T1 Q4
Tsys + T1+T2+T3 Q3
Tsys Q1
Tsys + T1+T2+T3+T4
Tx = Qx/Pitch radius of pulley

3. FEAD Basics More Belt Wrap = Higher accessory hubloads F=2T*sin(45)
BUT
Less likely to slip TT uB
TS e

4. FEAD Basics TT H B h Ts H = (TT2 + TS2 + 2TTTS*cos B)1/2
Tan h = (TT*sin B)/(TT*cos B + TS)
Where:
TT = Tight side tension
Ts = Slack side tension
H = Hubload
B = Wrap angle
h = Hubload angle

5. Relative hubload capacities of accessories
Alternator - low
Idler (6203 bearing) - low
Idler (6303 bearing) - low/medium
Idler (double row bearing) - medium
A/C compressor - medium
P/S pump - high
Water pump - high

6. Dynamic Accessory Torque
- All accessories and pulleys have a rotational moment of inertia, J. Additional torque is required to accelerate or decelerate the accessories during an engine transient condition.
Q = J*w
w = n*engine accel or decel rate
and since
Q = F*R, solving for F
F = J*w/R
Which shows that pulley ratio has a large impact on the Force required to rotate the accessory, since it affects both the numerator and denominator.
Where:
Q = Dynamic accessory torque
J = Rotational moment of inertia
w = acceleration of the accessory
n = Pulley ratio
F = Force applied at pulley pitch radius
R = Pulley pitch radius

7. The alternator is the most difficult component to drive
The mechanical fan is a close second
Accessory Typical Pulley Ratio Typical Inertia (lbm-in^2)
Alternator up to 3:1 up to 14.50
Fan 1.2:1 to 1.4:1 up to 200*
A/C :1 to 1.6:1 8 to 10**
P/S 1.2:1 6 to 8
Water pump 1.2:1 to 1.4:1 approx 8
*Fan inertia is not fully coupled to system due to viscous clutch
** Total inertia when clutch is engaged, less when clutch is disengaged

8. Automatic Belt Tensioners
Advantages of Automatic belt tensioners:
Belt tension is automatically set.
Belt tension remains relatively constant over time, belt does not loosen or have to be retensioned after mileage or time.
Belt tension does not have to be “over tensioned” to compensate for tension relaxation/belt stretch as on a fixed center or jack screw system.
Belt tension cannot be over tensioned during service or installation. Results in less load on accessories and better bearing and belt durability.

9. High mileage failure mode of belt is cracking/chunking
High mileage failure mode of belt is cracking/chunking. Small pulleys and higher span tensions are worse
Belt Material in Tension
Belt Pitch Line
Side view of belt bending around a pulley

10. Always package the tensioner on the slack side of the crankshaftQ2 Q4 Q3
Tsys Q1
Exit span of crank is lowest tension in system, needs to be controlled
for proper system function

11. In general, pulley in line with arm tensioners are better than offset pulley tensioners. Long arm tensioners are generally better than short arm tensioners.
Load on belt centered on pivot bushing
Load on belt offset from pivot bushing

12. Maximize tensioner belt take up
High wrap on pulley, long arm
Less tension loss with belt stretch and wear
Less tensioner arm motion and tensioner wear

13. Never, ever package consecutive backside pulleys

14. Spec belt length as short as possible for belt installation
Belt misinstallation warranty ranks in top 3 causes for FEAD
warranty. Short belt ensures that belt must be in the grooves
of all pulleys or belt will not go over flange of final pulley during
installation.
Tensioner arm angle at intended nominal angle, as tensioner
unwinds with longer belt, spring output drops

15. Long belt spans are better for alignment than short belt spans.
Large belt entry angle
Short Span
Small belt entry angle
Long Span
*Ford internal design requirement is 1/3 degree max statistical misalignment between pulleys using Variation Simulation Analysis.

16. Direct mount accessories to engine if possible
Direct mount accessories to engine if possible. Minimize stack across components. Keep sheave line as close to front face of block as possible.
Improved mounting stiffness
Improved alignment control