# Differential Gearboxes Ensure Lay Consistency for Wire Bunching EquipmentNovember 22, 2023

When it comes to twisting and bunching wire and cable, consistency is key. For example, the amount of insulation required for residential wiring depends on the wire’s lay — the distance between its twists, which is also known as the pitch. Without tight control during the wire bunching process, lay variations occur. As a result, the wire will require more insulation than necessary, raising its cost. A differential gearbox can drive rotating components in a single-twist buncher to provide a more uniform lay, using less power at a lower cost than a two-motor approach.

Effective Speed Control Is Critical to a Uniform Pitch
A typical differential drive on a single-twist buncher has two inputs and one output. When the reel rotates, wire is rewound. A rotating flyer twists the wire without rewinding. When both rotate together, the lay (or pitch) is determined by the following formula:

$Lay = {wire\ linear\ speed \over flyer\ rotation\ speed}$

The flyer rotates at a constant speed while paying off the wire. As the wire gathers on the reel, its diameter increases. The reel speed must decrease in order to achieve a constant linear speed and, ultimately, a uniform lay. Because the speed change occurs dynamically, a machine designer can choose to use two motors — one for each rotating element — or a differential and one motor.

One such differential-based system uses a main motor to drive the flyer and a Redex SR Series planetary gearbox to drive the reel. This system only requires input power for the wire, which is equal to the driving power minus the braking power.

The SR differential has a unique compound epicyclic gearing design that incorporates two sun gears within the casing along with two gears for each planet axis. Redex’s patented assembly process ensures equal load sharing between the planet gears: As planet gear assemblies are added, the torque capacity of the differential increases. The SR unit is designed and constructed to allow power to circulate through the differential so that one element is braking and the other element is driving. That means the differential input motor only needs to provide driving power for braking. Furthermore, there is a large ratio between the drive motor and the driven reel. Therefore, a large change in motor speed affects a small change in reel speed, which makes the speed control and, in turn, the lay very accurate during winding.

Boost Your Bunching Operation’s Bottom Line
The SR differential gearbox is also a more attractive option than a two-motor approach, which otherwise requires a designer to size and implement two large motors — one for braking power and another large motor for the driving power. Instead, the differential approach provides greater power efficiency and space use for the single-twist buncher plus greater control of the lay. The result: lower wire-insulation costs and a better bottom line for your bunching operation.