When adding truck-mounted equipment to vocational (work) trucks, it is essential to make the time for up-front considerations about needs, capabilities and options. The objective is to get equipment that meets a truck’s specific work requirements for its intended application.
Not doing so will end up costing more in the long run due to a loss of productivity. It could also affect the safety and convenience of the truck operators.
There are three important factors to consider in order to correctly specify a truck-mounted crane, say officials at truck crane manufacturer Liftmoore (www.liftmoore.com). They are:
- How much weight will be lifted, including all rigging used?
- How far out from the center of rotation will the load be?
- How often will the lift be made?
The maximum capacity of a crane is, as you would expect, the crane’s maximum lift capacity. However, Liftmoore officials do not recommend purchasing a crane for use at its maximum lifting weight.
While two cranes may be listed as 6,000-lb capacity cranes, their capacities may actually differ within the working load zones of the crane. Calculating the load moment capacity required, then comparing it to the load moment rating of the prospective crane, can eliminate problems down the road.
A crane’s maximum load rating is determined by the crane’s load moment or moment rating. This is the weight of the load being lifted multiplied by the distance between the load’s center of gravity and the center line of the crane.
The shorter the distance, the lower the load moment. That is why heavier lifts should always be done as close as possible to the crane.
TYPES OF CRANES
There are basically three types of truck-mounted cranes: electric, hydraulic and articulating, also known as the “knuckleboom.”
Electric cranes, which don’t require the engine to be running to power the crane, offer a low cost of entry, according to officials at Iowa Mold Tooling (IMT), a company that designs and manufactures mechanic’s trucks, lube trucks, tire trucks, air compressors and truck-mounted cranes (www.imt.com). Electric cranes are suitable for those applications where lifting needs are fairly light and duty cycle operations are intermittent.
They are not intended for extended continuous operation periods as constant use throughout an operation will dramatically reduce battery life and add pressure to the electrical system, officials at Auto Crane, a manufacturer of electric and hydraulic cranes, crane service bodies and accessories (www.autocrane.com), add.
Heat buildup can also become a hurdle during an electric crane’s operation, note officials at Service Trucks International, a builder of crane, service and lube truck bodies (www.servicetrucks.com). Electric cranes are limited to the duty cycle (heat generated) of the electric motors that power them. They work well if there is enough time between operations for the motor to cool to ambient temperature.
What’s more, the power of the electric motor typically limits the size of an electric crane to a 6,000- to 6,500-lb maximum lift capacity, they say.
Applications that require extended crane operation periods, shorter cycle times and higher lifting capacities are best served by using a hydraulic-powered crane, say Service Trucks International officials.
Hydraulic cranes require a power take-off and pump configuration to produce the necessary fluid-filled hydraulic system flow to power the crane, Auto Crane officials explain. This requires the engine to be running, often at higher than idle rpm, during operation.
Hydraulic cranes also offer greater control (proportionality) of the crane and faster operation, and hold up to higher cycle counts better than electric cranes, they add.
Hydraulic telescopic cranes have a wide enough range of lift and reach capabilities to meet most field service needs.
The cranes, however, have a lift capacity limitation because of the service bodies they are mounted on, IMT officials say. The size of a crane and the size of the truck go hand-in-hand.