This article contains information regarding push pull cable controls and their use.
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A push-pull cable control is designed for precise multi-directional mechanical motion control. It comes in various types, adaptable to a wide range of applications. Push-pull assemblies are commonly used in the automotive, aerospace, and heavy equipment industries for adjusting and maneuvering mechanisms.
These cable controls consist of two distinct cables: one activates a device while the other deactivates it. Despite the name “push” cable, it is actually a pull mechanism, similar to the other cable, which also pulls.
While push-pull assemblies serve a straightforward function, they play a crucial role in the design and engineering of mechanical components and analog systems. For activating equipment through physical force, a push-pull control cable is often the preferred choice.
The design process for push-pull assemblies starts by evaluating the workload during the push mode. The inner wire may encounter resistance, potentially causing it to bend or buckle, especially when extended from the conduit. To address this issue, increasing the diameter of the inner wire can help accommodate the workload.
Engineers must consider various factors, including the behavior of the inner wire in push mode, when designing push-pull assemblies. Although the basic operation of these assemblies is simple, precise and accurate calculations are essential to ensure their effective performance.
The design and manufacturing of push-pull assemblies are essential for effective machine design. A well-engineered push-pull assembly ensures that powered functions operate smoothly and have an extended lifespan. These assemblies, also known as control cable assemblies, manage various functions such as acceleration, actuation, braking, choking, drive latches, hood releases, rotation, and throttling, among others.
In the design phase of a push-pull assembly, meticulous attention is given to each component, as defects in any part can significantly impact overall performance. While there are various types of push-pull assemblies, certain components are common to all types.
The casing of a push pull assembly, also known as the conduit, serves as a guide for the inner core. There are a variety of materials used to manufacture the casing, including galvanized steel, nylon, plastic tubing, or spiral wrapped wire. The casing is sufficiently flexible to allow the control system to be bent to meet routing needs. Various types of fittings can be added to the casing to increase the ease of operation.
Casings feature an inner diameter designed to house the cables and an outer diameter that provides the necessary strength and durability to safeguard the mechanism.
The bulkhead terminal is crimped onto the casing to firmly attach the push-pull control to a panel, bulkhead, or solid part of a mechanism. Typically, bulkhead terminals are constructed from steel, stainless steel, or brass.
Various options exist for the inner wire of a push-pull assembly. Even for light loads, a robust wire is necessary. The choice of wire is influenced by the pull position, where loads are most significant. Typically, cable wires are crafted from stainless steel or galvanized steel due to their strength, durability, and dependability.
Flexibility is also a key factor for the cable wire. Most assemblies use 7 x 19 or 7 x 7 wire configurations due to their superior flexibility, while 1 x 19 wire, though less flexible, is used in some cases.
Friction inside the casing or conduit can lead to wear on the assembly, so uncoated wire is preferred over coated wire. This choice helps prevent damage to the assembly, extends its lifespan, and reduces the risk of failure.
The travel distance, or the span from the end of the conduit to the cable's bearing point, should be minimized. Longer travel distances increase friction and decrease performance, potentially leading to core buckling. Ideally, travel should be kept to under five inches.
Inner wires or cables are wound in a uniform helical pattern to form a strand. Strands typically consist of seven to nineteen wires wrapped around a core or central strand. In strand notation, the first number represents the number of strands, and the second number indicates the number of wires per strand. For example, a 7 x 7 cable has seven strands with seven wires each. Cables with more strands and wires offer greater flexibility.
Solid core wire is used in applications requiring movement in both push and pull modes. It is also employed when a strand or cable needs additional stiffness to effectively actuate the process.
Fittings and terminals are crucial components attached to the end of the conduit to secure the push-pull assembly. They help prevent deflection under tension, which can lead to system failure or loss of control. These components are essential for adjusting, actuating, accelerating, regulating, and releasing mechanisms, ensuring both precision and stability.
Stamped eyes are a frequently used type of fitting, available in various sizes to match the cable dimensions and the eye hole. They are securely attached using hydraulic swagers, ensuring a tight and reliable connection.
Ball end fittings are available in multiple sizes to accommodate various cable diameters and are swaged onto the end of the cable for secure attachment.
Ball shank fittings are employed in push-pull assemblies for applications like throttles, braking systems, and exercise equipment. They shield cables from abrasion by other metal components and enable smooth cable rotation.
Thimbles are employed when creating loops in cables. They help prevent wear, maintain the shape of the loop, and bolster the cable's strength. As a robust type of end fitting, thimbles provide essential protection for the cable.
Threaded studs are used to secure a push pull assembly to a fixture. They come in various sizes to accommodate different applications and are typically swaged onto the cable, similar to other fittings.
Strap fork and eye fittings are used for pull actuation with pin or bolt attachments. A strap fork allows direct actuation between the forks, while a strap eye is employed when a rigid or straight actuation is needed. These fittings are often welded together to create a thinner profile, making them suitable for tight spaces.
Marine eye fittings, designed for the boating industry, are made from grade 316 stainless steel to withstand harsh marine environments. These fittings offer a shiny, appealing finish and are highly durable and resilient. Their corrosion-resistant materials and specialized treatments contribute to their higher cost.
Handle and knob fittings are commonly used in lawn mowers, chainsaws, and other garden equipment. They serve as actuators for various functions, including adjustments, rotation, release, and acceleration.
The end fittings, terminals, caps, handles, and knobs mentioned above represent just a few of the many types available. Specialized push pull controls feature connections and fittings tailored to meet the specific requirements of their applications.
Push pull cable controls have diverse applications across numerous industries, from complex aerospace systems to everyday gardening tools. They are engineered to manage processes, adjust mechanisms, and activate various components during operation.
With the advent of the computer age, it could be assumed that push pull cable controls would no longer be necessary. In actuality, push pull control cables have become essential for programming automated equipment. The versatility and adaptability of push pull controls and their ability to perform accurately and flawlessly have made them ideal for the many computer run functions.
Modern aircraft demand reliable components to ensure safe control and optimal performance. Push pull cable controls enable remote activation of aircraft mechanisms, providing the necessary precision and dependability.
Aircraft components must withstand high tension and compression forces experienced during flight, making push pull cable controls an ideal solution. For aircraft applications, these controls are typically manufactured in short lengths to minimize vibration and prevent bending under compression.
Push pull controls are widely used in boats to mechanically connect the control head to the clutch and throttle. They are especially effective for boats with one or two control stations and cable lengths of less than 50 feet. In a single-lever control system, pushing the lever forward engages the clutch, while continuing to push increases the speed. Pulling the lever back reverses these actions, reducing speed and disengaging the clutch.
In lawn mowers, push pull control cables function as throttle cables to regulate the engine speed. Modern lawn mowers connect these cables directly to the carburetor's intake valve. The maximum push load for a throttle cable is 10 lbs, while the pull load can reach up to 25 lbs.
Heavy duty equipment, such as trucks, rely on push pull control cable assemblies to activate several operations and processes. There are cables for front winch control, dump body control, shift control, transmission engage control, defrost control, engine stop, and throttle control, to name a few. These different push pull control cables make operating heavy duty trucks easier and more efficient.
The examples of push pull control cables provided here represent just a small fraction of their many applications. These cables are utilized in industrial machinery, automated systems, construction equipment, and numerous other fields. Their ability to activate and control processes with accuracy and precision is what makes them so versatile and widely used.
Push pull control cables are at the heart of motion control systems. In the production of cable assemblies, certain factors have to be considered such as the workload in the push and pull modes, where the workload in the push mode is the most critical. Other design considerations are abrasion, cycle life, flexibility, the environment, and safety.
The versatility and reliability of push pull cable controls often make them a preferred alternative to electronic, hydraulic, and pneumatic systems. Various installation methods are employed for push pull control cables, depending on the specific application, structural considerations, and accessibility.
The most widely used installation method for push pull cables is the rivet technique. This usually involves placing two rivets at right angles at each end of the mechanism. Special attention is needed when dealing with rod ends that have hollow shanks. Depending on the situation, rivets may be inserted with a rivet gun or manually hammered into place.
Securing push pull cable assemblies using bolts and nuts involves selecting bolts that can handle the assembly’s tension and compression. The size and strength of the bolts are chosen based on the dimensions of the push pull assembly, with larger assemblies needing more robust bolts. Typically, fiber or nylon lock nuts are used to secure the connections, as they provide locking capabilities to prevent loosening due to vibrations.
Welding push pull cable controls is a method applied to assemblies constructed from steel or stainless steel. This technique provides a secure attachment of the assembly, though it demands a high level of expertise and careful attention to ensure accurate placement and proper fit.
For shorter push pull cable controls, particularly those around eight to nine inches in length, threaded fittings are highly effective. This method enables adjustments at both ends of the rod, although typically, only one end remains fixed. The key benefit of adjustable ends is the flexibility to make a broad range of modifications and fine-tuning.
When dealing with push pull cable assemblies, calculating the workload is crucial for ensuring the mechanism functions correctly. This factor represents the force needed to operate the device.
The primary focus is on the pushing mode, which handles a heavier load. Adjusting the cable diameter can accommodate increased workload in the push direction. On the other hand, the pulling workload is more adaptable and can be adjusted with greater ease.
Backlash refers to the motion lost during the actuation of a push pull cable. This factor is typically considered during the design stage but can be adjusted by increasing the number of bends and tightening the bend angles. Reducing the number of bends and angles helps minimize backlash and improves overall performance.
Each push pull control cable installation is tailored to its specific application. The four connection methods discussed are just a few examples. The installation approach depends on the types of fittings used and the installation location of the cable.
Considerations such as backlash, vibrations, abrasions, and workload are relevant to all push pull control cable installations and are usually addressed during the design process. However, these factors should also be evaluated during the assembly's operation. The most important aspect when selecting push pull controls is the quality and reputation of the manufacturer, which can significantly mitigate future issues.
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