Travel. Experts recommend 5-in. maximum travel for most light and medium-duty push-pull applications. This minimizes lost motion and potential for the core to buckle. Use even shorter travel lengths with small-diameter cores.

Travel in pull-pull applications has fewer restrictions and can generally exceed 5 in. However, if the core is subject to hostile environments, minimize the stroke to limit exposure outside the conduit.

LOST MOTION

Perhaps the least-understood design factor is lost motion. All push-pull controls lose some motion between input and output sides when applying a load to the system. Lost motion increases with higher loads, more bends, and longer assembly lengths. It can be overcome by designing over travel into the system at the input or output ends, or both.

Lost motion results from deflection and backlash.

Backlash is caused by the clearance between the core OD and conduit ID, and is present in both push and pull operating modes. It is proportional to the number and length of bends in the installed assembly, and the clearance between core and conduit. Calculate backlash B using:

B = X R2/180 X R1/180

where R1 = centerline of core in tension (no load); R2 = centerline of core in compression (no load); and X = total angular degrees of bend in the routing.

Deflection comes from elastic strain caused by tension and compression loads on the control. Calculate deflection ΔL as:

ΔL = FL/AE

where F = average force, or one-half output load + one-half input load; L = length of active inner core; A = core cross-sectional area; and E = the core's modulus of elasticity.

Note that actual deflection of a control in compression may vary from calculated values based on the column strength of the core and conduit and the buckling potential. And lost-motion calculations assume the control is securely mounted on the ends and the conduit is firmly held in its routed position.

Lost motion is also a factor in pull-pull controls. They typically have little backlash because they operate under tension. However, these controls are subject to the same deflection factors as push-pull controls. And routing always affects the travel length. In some cases, lost motion can be accurately calculated. In others, installing a prototype in the system to confirm correct design length and travel is highly recommended.

FRICTION CONSIDERATIONS

Efficiency. The conduit, core, and number of bends, as well as relative friction between core and conduit, determine a push-pull control's efficiency. Depending on the materials, good practice is a 2 to 10-in. minimum bend radius. Estimate the minimum bend radius by multiplying the core diameter by 100.

Bends in the system create friction and reduce efficiency. Estimate frictional effects from:

I = P f

where I = actual input load; P = output load; and f = input load factor, found in the accompanying graphic. Percent efficiency is then determined from:

η= (P/ I) 100.