Carl Stahl Sava Industries, Cable Assemblies,



Push-Pull Control Assemblies


Push-Pull Control
Assemblies

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Push-Pull Control »
Design Information

Push-pull and pull-pull cable controls offer a reliable method of transmitting motion between two fixed points or between points which are changing their relative position. Because of flexibility, they can be routed up, down, over obstacles and around corners without intermediate links or pulleys. Fewer working parts increase operational dependability of cable controls. They are virtually maintenance free as no periodic adjustments are necessary due to wear and tear of worn connections. Cable controls do not transmit noise and vibration.
SAVA is flexible enough to handle small as well as large volume orders for cable controls. A wide variety of end fittings are available to the designer for use with the casings and core cables.

Construction

The basic component of a push-pull control consists of a solid wire with a casing of plastic tube or spirally wrapped wire. See Figure 1. Substituting a flexible cable for the solid wire allows the control system to be bent to facilitate routing.
Different fittings as shown in the following text can be attached to the ends of the casing and cable for ease of operation.

Design Information - Figure 1
Figure 1

Design Information - Figure 2, Lost Motion, Tension Load, Compression Load
Figure 2

Loss of Motion

The principal elements of lost motion in a control system are backlash and deflection. See Figure 2. Backlash is caused by the core member moving inside the casing with the change in direction of motion. It is a function of the clearance between the core and casing and total number of degrees of bend in the cable. This can be reduced by careful design. The other cause of loss of motion is deflection of the core wire under compressive load. Elastic strain in the core member due to compressive or tensile force also contributes to the loss of motion. The casing must be anchored securely to keep it from responding to the compression or tension modes of input loading.

Travel

Travel of the core inside the casing should be kept to a minimum since longer travel increases friction and decreases output. In the push-pull type of application, the chance of buckling of the core becomes greater. The travel should be limited to less than 5˝ if possible. The linear speed of operation should be relatively low.

Bend RADII and Life

Cable bend radii should always be as generous as possible for maximum cable life and efficiency. Smaller bends cause reduced service life because of added friction. Depending on the size of the casing and the construction of the moving core member, the minimum recommended radius can vary from 2 to 8 inches.

Input Load Factor

Friction between the core and the casing causes a loss in output force for a certain amount of input force. Friction is a function of the degrees of bend in the system. The ratio of the input force to the output force is called the Input Load Factor. The Input Load Factor has been plotted against the degrees of bend in the system and is shown in the accompanying graph. For selecting the right control system, the input load has to be determined by multiplying the output load with the Input Load Factor obtained from the graph using the following formula:

Input Load Factor


I = Input Load
P = Output Load
F = Input Load Factor (from graph)
I = P x F
Example: Consider a push-pull assembly with metal-lined casing requiring an output load of 6 lbs. Total degrees of bend in the system–270°. Input Load Factor from chart–2.05.
Input Load = 6 x 2.05 = 12.30 lbs.