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*ww = n*engine accel or decel rateand sinceQ = F*R, solving for FF = J*w/RWhich 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 torqueJ = Rotational moment of inertiaw = acceleration of the accessoryn = Pulley ratioF = Force applied at pulley pitch radiusR = Pulley pitch radius
7 The alternator is the most difficult component to drive The mechanical fan is a close secondAccessory Typical Pulley Ratio Typical Inertia (lbm-in^2)Alternator up to 3:1 up to 14.50Fan 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 8Water 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 worseBelt Material in TensionBelt Pitch LineSide view of belt bending around a pulley
10 Always package the tensioner on the slack side of the crankshaft Q2Q4Q3TsysQ1Exit span of crank is lowest tension in system, needs to be controlledfor 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 bushingLoad on belt offset from pivot bushing
12 Maximize tensioner belt take up High wrap on pulley, long armLess tension loss with belt stretch and wearLess tensioner arm motion and tensioner wear
14 Spec belt length as short as possible for belt installation Belt misinstallation warranty ranks in top 3 causes for FEADwarranty. Short belt ensures that belt must be in the groovesof all pulleys or belt will not go over flange of final pulley duringinstallation.Tensioner arm angle at intended nominal angle, as tensionerunwinds with longer belt, spring output drops
15 Long belt spans are better for alignment than short belt spans. Large belt entry angleShort SpanSmall belt entry angleLong 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 stiffnessImproved alignment control