Hydraulic hooklift hoist

The hydraulic operating pressure of all hooklift hoists are preset at the factory to achieve the intended lifting capacity for the design application. The original approach used by early European models, still widely used today, was a high pressure / low volume system. This setup suits chassis with space restraints, allowing for use of a smaller piston pump and a smaller hydraulic reservoir. Such systems operate between 4,000 to 5,800 psi (28,000 to 40,000 kPa).

The North American market developed low pressure / high volume systems, which allowed operators to share the hoist's hydraulic system with other hydraulically powered devices. These systems typically operate with a larger gear pump and larger hydraulic reservoir. Operating pressures range from 2,000 to 3,500 psi (14,000 to 24,000 kPa).

There are two common types of jib system, each controlled by a single hydraulic cylinder. Both systems can be used with either a single or dual rear pivot section.

• The articulating jib, also referred to as a tilting jib, is a simple a-frame. When extending the cylinder, the container/body is pushed up and back, exiting the rear locks. When retracting the cylinder, the container/body is pulled forward and down, engaging the rear locks.
• The sliding jib consists of a single arm, composed of a tube inside a tube. When extending the cylinder, this design pulls the container/body forward, engaging the rear locks. When retracting the cylinder, this design pushes the container/body rearward, exiting the rear locks.

• The single rear pivot system is the simpler of the two rear pivot designs. Every function operates relative to a pivot point, typically located in front of the rear axle. This design does not allow traditional tip-dumping capabilities.
• The dual rear pivot system is more complex, offering the added benefit of a traditional tip-dumping capability. This is achieved by pivoting at the rear of the hoist, typically behind the rear axle.

25-ton hooklift hoist (dual lift/dump cylinder) mid lift

There are typically two styles of rear lock. Both styles can be located on the hoist such that they are positioned either on the inside or outside of the container/body long rails.

• The prong style rear lock offers a container/body engagement of up to 7 in (178 mm). This design allows restricted weight transfer while the container/body remains inside the rear locks.
• The slide through style rear lock offers a container/body engagement of up to 50 in (1,270 mm). This style typically allows more extensive weight transfer while the container/body remains inside the rear locks.

Two configurations are typical, both suitable for either single or dual pivot designs.

• The single lift/dump cylinder design reduces unit cost, retains true hooklift capabilities, but can be unstable while dumping on uneven ground.
• The dual lift/dump cylinder design, whilst increasing unit cost, improves load handling stability when dumping on uneven ground.

A series of safety valves act to ensure safe operation of the system.

• Counterbalance valves, also called load holding valves or over-center valves, serve as hydraulic resistance to the actuating cylinder when the load weight is required to be held in position for a period of time. These come in two distinct configurations, both highly effective in improving load handling safety during a loss of hydraulic pressure: essentially locking the lift/dump cylinder in-place, preventing potential free fall of the load in the event of hydraulic failure.
  • Remote mounted system, in which the counterbalance valve is mounted outside the cylinder. This is the more cost effective solution, but can be prone to damage.
  • Integral system, in which the counterbalance valve is built directly into the main cylinder, eliminating the potential for damage by falling debris.
• Jib safety valves are not a feature of all systems, but are designed to render the jib cylinder inoperable during the dump cycle. This protects the system from damage should the operator accidentally pull the wrong control lever during the dump cycle.
• The hook latch covers the opening in the hook head, effectively securing the lift bar inside the hook head during transport and dump. It is designed to augment the rear lock system during transit
• Some systems utilize proximity switches to indicate to the operator which part of the lift/dump cycle is current, or whether there is a fault in the system. These can me mounted in a variety of locations throughout the hoist. When activated, a red warning light and/or alarm sounds in the cab.
• Below grade reach is the distance that the hook head travels below the lifting bar on the container. This distance varies between hoist systems, ranging from 1 to 24 in or 25 to 610 mm. A below grade reach of 5 in (127 mm), or greater, improves stability when working on uneven ground.

Cab controls can either be operated by cable, air or electricity. Both cable and air controls, in a floor mounted lever configuration, offer additional benefits in controlling the load, by feathering the controls.

• Cable controls are the least expensive, and are commonly floor mounted levers. They are generally favored in cold weather environments.
• Air controls can be installed as floor mounted levers or dashboard mounted switches.
• Electric controls are the most expensive and only come as dashboard mounted switches. The disadvantage of these systems, is that the control is either on or off, and thus controlling the load by feathering is not an option.