A winch or hoist unit (often called drawworks on some rigs, including oil and gas rigs) usually consists of a drive unit, a drum, and a braking system.
The ability of the hoist drive to lift a load depends on the torque available.
When the available torque is limited, the maximum line pull available depends on the efective diameter of the winch.
An empty drum has the smallest efective diameter and can provide the largest pull.
As the drum becomes full, the efective diameter approaches maximum and the line pull reduces.
Thus the hoist single line pull is controlled by the:
- drive torque
- efective drum diameter
- size of rope
- length of rope (governs or limited by drum size).
The hoist unit must be equipped with a brake that will safely hold the design load and retain its holding capability even when providing continuous braking effort.
Braking is provided by friction bands working on a drum (or pads on a disc). If anything interferes with the friction force (e.g. wet or oily drums) then the brake capacity is reduced.
Overheating of the brake lowers the friction force.
Disc brakes use a lat, disc-shaped metal rotor that spins with the hoist drum. When the brakes are applied, a calliper squeezes the brake pads against the disc.
Figure 1: Disc brake schematic
Figure 2: Disc and calliper
Disc brakes are generally considered superior to drum brakes for several reasons:
- They dissipate heat beter (because brakes work by converting motion or kinetic energy to heat energy).
- Under severe usage, such as repeated hard stops or riding the brakes for long periods, disc brakes take longer to lose efectiveness. (This reduced efectiveness is known as ‘brake fade’.)
- Disc brakes also perform beter in wet weather, because centrifugal force tends to ling water of the brake disc and keep it dry, whereas drum brakes will collect some water on the inside surface where the brake shoes contact the drums.
The breaking capacity depends on the:
- Diameter of the disc or brake drum
- Number of discs or brake drums
- Kinetic coeicient of friction between the brake pads and the disc or drum
- Ability to dissipate heat caused by the friction.
The brake must be able to hold the weight of the drill string as well as dissipate the kinetic energy of the drill string when it stops the moving string. This energy is dissipated as heat.
A typical hydraulic winch is shown below in Figure 3. It incorporates a hydraulic motor, a multiple-disc brake, internal reduction planetary gears, and a winch drum that holds the rope.
Figure 3: Hydraulic winch
Positive displacement hydraulic motors can assist the braking action.
Multiple-disc brakes are very good at holding the load.
Planetary gears increase the torque of the winch and hence the pulling power.