Control Cables Design Guide
Push-Pull and Pull-Pull Controls Design Guide
There are many common, everyday applications that use efficient and reliable CMA controls. These applications include
automotive seat releases, lawn mower throttle controls, and airplane seat back controls. Some uses of controls are obvious and visible, while others are not. Simple and lightweight controls for dependable, remote activation of throttles, gas springs, mechanisms, or electromechanical devices, etc., can use either push-pull or pull-pull type products.
Solid core controls are used in applications that require the transmission of forces in both the push and pull mode of operation. The vast majority of these products use various combinations of conduit styles and solid wire cores in their construction.
In some cases, stiffer, small diameter cables can be used for push-pull applications, providing the push load is very light and the combination of cable and conduit is carefully considered. Solid wire cores can be formed on the ends to eliminate the need to apply separate fittings or terminations. Bend radii should be large and routing must be simple in order to avoid a permanent set in the core wire. All push-pull controls have a larger capacity in the pull (or tension) mode than in the push (or compression) mode.
Flexible core controls are used in applications that require the transmission of forces in tension only. In general, more flexible conduit and a cable core is used for greater freedom in routing and smaller bend radii in restricted installations feature an integral return spring to keep a specified load on the cable and to return the cable or mechanism to its original position after activation. Controls with cable cores permit the use of most standard fittings allowing a very wide range of mounting and retaining options.
Push-Pull/Pull-Pull Control Design Factors
Applications — Analysis and review of the proposed control function will determine what type of product can be used for a given application. Consideration and definition of potential system variables, such as load, routing, friction, stretch, set, effects of bends on length, temperature, environment, and exposure to containments, will aid in the design of an acceptable control. In general, pull-pull controls are lighter in weight and more flexible that push-pull controls, however, this is a function of the application and load requirements.
Push-Pull — Working loads should be specified as the highest in the pull or tension mode, with the push or compression mode specified at 50% or less of the pull mode. Lower working loads in the push function minimize the tendency of the core to displace the conduit and, more importantly, reduces the potential for the unsupported core outside of the conduit to kink, bend, or distort.
NOTE: The final specifications should be based on the travel core material column strength and the type of use.
Consult CMA’s Engineering department for additional information.
Pull-Pull — Maximum working loads should be specified based on the minimum breaking strength of the cable with a safety factor, and the resistance of the type of conduit selected to resist deflection and compressive forces when a load is applied to the core. Extremely high loads and high life cycles can cause the cable to stretch and wear through the liner of the conduit. The conduit and retained in its routed position for proper function.
Push-Pull — It is recommended for most light-and medium – duty applications that travel should be limited to 5″ maximum. This
minimizes possible loss of input versus output, and potential for the core to buckle. In very small diameter core situations, a smaller
travel should be used to minimize the possibility of buckling. Each application should be analyzed to compare travel and the applied
load to determine if core buckling could occur.
Pull-Pull — Travel of cable cores can usually be specified to suit the application. If the control and core are subject to a hostile
environment that could affect the core, a minimum stroke should be specified to limit the exposure of the cable outside the conduit.
NOTE: Travel and design length of the core is affected by the total number of degrees of bend in the actual routed
installation. The type of conduit, core and clearance between the O.D. of the core and I.D. of the conduit can increase or decrease
the travel length of the core member. It is highly recommended that a prototype be installed in the system to confirm correct
design length and travel.
Push-Pull — Consideration must be given to the fact that all push-pull controls are subject to the loss of motion between the input
and the output ends when a load is applied to the system. Total lost motion of the control consists of backlash and deflection.
Backlash is the lost motion caused by the clearance between the core diameter and the inside diameter of the conduit. It is
present in both the push and pull modes of operation. Backlash is directly proportional to the total degrees of bend in the installed
routing and the clearance between the O.D. of the core and the I.D. of the conduit. It can be calculated as shown below:
The total amount of lost motion increases with higher loads, more bends and increased length of the control. It can be overcome by
designing over-travel into the system at either the input or output ends, or both.
All consideration of lost motion must include the assumption that the control will be securely mounted on either end, and the
conduit is firmly held on its routed position.
Pull-Pull — Lost motion is also present in pull-pull controls. Backlash is minimized since most pull-pull controls function under
tension. The length of the travel in any cable control will be affected by routing. The required travel for actuation should be added
or subtracted to the amount of travel lost or gained in the routed positions In some cases, this can be calculated; in others, samples
installed in the system are more accurate in determining core length.
However, these controls are subject to the same deflection factors as push-pull controls. Deflection should also be calculated
and used as a design factor.
Alignment and Installation
Correct mounting and careful alignment of the cable control and core can maximize efficiency, decreasing working loads and
increase cycle life. All controls should be securely mounted and installed to keep cable or core travel in a straight line to the load or
device being actuated. In cases where a lever arm is being moved, mount the control to minimize angular deflection of the core, as
illustrated. Specify a fitting or assembly detail that will allow rotation at the mounting point.
Push-Pull—The conduit and core selection, as well as the number of bends in the system, combined with the relative friction
between the core and the conduit, determine the efficiency of the control. Depending on the materials selected, bends should be
kept within a 2″ to 10″ minimum radius. Recommended minimum bend radii can be estimated by multiplying the core diameter
“D” x 100.
Efficiency is reduced by the friction created by bends in the system. This can be calculated using the graph and formula below:
Pull-Pull — Cable cores can create greater friction in either a lined or underlined conduit, and therefore, cable construction should be considered. Most cable controls use 1 x 19 cable, since it has the smoothest outside diameter, and is more flexible than a solid
core. For applications requiring more flexibility, 7 x 7 cable can be specified. However, if higher loads are involved, efficiency is
reduced and the liner can be subjected to undue wear and damage.
Lubrication is not recommended for most applications. Lubricants can decrease the efficiency of lined conduit and only offer
minimal improvements in unlined conduit. Most lubricants tend to collect dirt, dust, etc., that can build up on the core and decrease
its efficiency. Special combinations of swaged cable and conduit with liners of very low coefficient of friction are available. Consult
CMA Sales and Engineering for further details.
Shuttle Molded Cable Contols
All design considerations for push-pull controls also apply to molded cable controls. Additional design factors do affect the design
of these products. They include the molded conduit fittings, conduit selection, assembly functions and dimensions.
Recognition of these factors, combined with a practical design for the application, will result in a successful, reliable and
Shuttle Molded Conduit Fittings
Accepted industry standards for molded part design should be utilized in the design and development of a custom molded fitting.
In general, the following should be considered:
- Determine the type of assembly option desired; i.e., snap in place, push in place, retained with fastener, etc.
- The wall thickness should be as consistent as possible to minimize potential warpage or excessive shrinkage.
- The design should permit a minimum of 1 /2″ length of conduit to be molded within the part. Using notched or serrated conduit with 1 /2″ minimum length results in a minimum of 35 lbs. pull-off.
- The design should allow for a straight “open and close” of the mold. NOTE: Our Engineering personnel will evaluate your custom conduit fitting and make recommendations regarding its moldability.
- The fittings must be designed to allow the insertion of a support pin into the inside diameter of the conduit.
Material selection should be based on temperature, strength and flexibility requirements. Almost any commercially available
thermoplastic resin can be shuttle molded. Typical production parts are molded from the following resins.
|Polypropylene||Low cost, flexibility, low strength and lower temperature ranges||Handles and bushings|
|Acetal||Moderate cost, good strength, lower impact, moderate stiffness and temperature ranges||Standard fittings, ferrules, bushings and flags|
|Nylon 6||Some flexibility, good impact resistance and higher temperature ranges||Conduit fittings, ferrules and bushings|
|Nylon6/6 Glass Reinforced||Higher stiffness, good strength and higher temperature ranges||Bushings, levers, brackets, handles|
NOTE: Standard colors for these materials are black or natural. Custom colors are available at additional cost.
Select the material based on the application, with the following criteria considered:
- Environmental and operating temperature ranges and extremes
- Axial and side loaded requirements
- Exposure to chemicals or containments
- Assembly method to mating part or mounting surface
- Actual part function
Conduit and Core Selection
The size and type of the core (cable or wire) and selection of the conduit is determined by the application.
The majority of applications utilize braided, reinforced conduits. It is also possible to shuttle mold conduit fittings on a flat
wire Bowden, as well as long lay conduit (consult CMA Engineering for further information). Braided conduit is coated with either
polypropylene (relatively stiff) or nylon (more flexible and heat resistant). Refer to the chart on conduit constructions on page 8 to
aid in selection of the appropriate material to meet your needs.
All conduit and core combinations should include a reasonable clearance between the inner I.D. and the core O.D.
For most light and medium duty pull-pull applications, a clearance of .015 to .025 between the conduit I.D. and the cable O.D.
*NOTE: Decreasing the clearance to minimize the effects of lost motion can result in dramatically higher operational efforts.
*Based on a comparison of the same diameter cable made from the same wire material.
Conduit Selection and Description
Shuttle Molded Controls
Cable Assembly Tolerances
|Assembly Length In Feet||Tight Tol.+/- in./mm||Normal Tol.+/- in./mm||Relaxed Tol.+/- in./mm|
|>50 ft.||Dictated by Application - Contact CMA|
NOTE: Tolerances tighter than shown are sometimes attainable at additional cost. Contact CMA Engineering for additional information.
NOTE: When dimensioning your finished drawing it is recommended that you specify the conduit overall with the cable/core overall
OR the conduit overall and the travel.
Additional specifications, which should be added to your assembly drawing.
- Holding strength of cable/core end fittings to be—lbs. (N) min.
- Holding strength of conduit end fittings to be —lbs. (N) min.
- Max working load in pull mode to be —lbs. (N).
- Max working load in push mode to be —lbs. (N). (Solid Core Assemblies)
The process permits the molding of almost any type of thermoplastic component directly to the surface of conduit, cable or wire.
Shuttle molding is done with a vertical injection molding press that utilizes a shuttle or table that moves back and forth in front of the injection nozzle. The upper half of the molds mounted to either side of the shuttle table. With the press closed and in the molding cycle, one lower-half mold is positioned to either the right or left of the head. This permits the operator to remove completed parts and to reload cut lengths of serrated conduit in preparation for another molding cycle. The process offers the following advantages:
- More cost-effective than assembled steel or plastic components.
- Manufactured and assembled in one operation.
- Better pull-off strength than conventional assembly techniques.
- More design flexibility than metal conduit fittings and flags.
All molds are made using standard injection mold tool steels. All cavity inserts and cores are hardened to specification, to reduce
wear and insure tool life. Most shuttle molds are made in 1, 2, 4 and 8-cavity configurations. The number of cavities is dictated by:
- Part size.
- Monthly or annual production volume requirements.
- Piece part vs. tooling cost analysis.
- Complexity of the part design.
Custom designed conduit fittings can be prototyped using a “soft” single cavity and core pin inserts in our prototype mold base.
Prototype tooling can prove the design concept and aid in final specifications for the cable control system. Made for low volume
(500-1,000 pieces), these tools are the best way to test a design and get a program going.
CMA’s extensive selection of tooling for conduit, cable, fittings, as well as standard molded conduit fittings, offer you many
solutions for your cable control needs.
Our Engineering personnel can offer suggestions and design assistance early in your cable control project. Call us with your
Cable Controls and Remote Actuation Products
CMA offers the widest selection of products available for a multitude of remote actuation requirements. Cable controls offer
lightweight, reliable and cost effective solutions for the remote actuation of latches, releases and locking gas springs. These
products are being used in many markets that include furniture, physical fitness equipment, recreational vehicles, transportation
and specialty seating, medical furniture and equipment and custom OEM products.
CMA will assist you in the design and specification of the best assembly for each application. Where it is required and the
volume justifies, we will assist in the design of custom components unique and proprietary to your requirements.
Interested in a PDF of the Control Cables Design Guide?
Interested in Other Design Guides?
Download our other two design guides for further information!