FEAD Basics F = Q/R TT = F + TS TT Q=Torque Q=FxR Ts Where:

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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

Accessories downstream from the crankshaft will have higher hubloads
Q2 Tsys + T1+T2 Tsys + T1 Q4 Tsys + T1+T2+T3 Q3 Tsys Q1 Tsys + T1+T2+T3+T4 Tx = Qx/Pitch radius of pulley

BUT Less likely to slip TT uB TS e

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

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

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

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

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.

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

Always package the tensioner on the slack side of the crankshaft
Q2 Q4 Q3 Tsys Q1 Exit span of crank is lowest tension in system, needs to be controlled for proper system function

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

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

Never, ever package consecutive backside pulleys

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

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.

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

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