Hydraulics are predominantly used in most mobile crane designs, from the cylinders for lifting, to valves for holding, and more.
By: Josh Cosford, Contributing Editor
We’ve all seen those crane yards when driving along highways near towns and cities. You know, the ones with their booms reaching high into the sky, proudly advertising the location of an equipment rental yard. They’re the industrial equivalent of the wacky waving inflatable flailing tube man. As much as my hydraulic sensibilities are offended by the apparent disregard for safety by leaving those booms elevated, I’ve never heard a story of one crashing down and hurting anyone.
Mobile cranes are one of the lesser-discussed hydraulic machines, although they’re often equipped with some of the largest hydraulic cylinders this side of a forging press. Many machines are fully hydraulic, while unique animals like tower cranes may use electric motors for some functions. A crane is a machine that simply lifts or lowers objects too heavy for simpler means. They shouldn’t be confused with equipment such as telehandlers or scissor lifts, which don’t use cable and winch systems. It’s typical to use a telehandler for lifting purposes, but fixed-length straps are used rather than winched cables.
The appearance of a mobile crane differs mainly by the terrain in which it travels. Truck-mounted cranes span from small units mounted to light-duty service trucks and grow to enormous, specialized machines with rows upon rows of tires to support their many thousands of tons combined vehicle and lifting weights. Truck-mounted cranes are used for road-legal on-highway machines, although many are rated for all-terrain use.
If your worksite duties are extreme, the track-mounted crawler crane is difficult to sideline in even the worst ground conditions. Although they look much like an excavator, they are outfitted with booms, winches and counterweights. Their long and wide footprint provides a stable crane base without the need for outriggers, as many other cranes require.
Covering the lighter-duty side of mobile crane technology are the carry deck and pick-and-carry cranes. These are smaller and highly mobile cranes with plenty of crossovers, primarily differing only where the pick-and-carry crane is meant to transport the load while the carry deck crane remains stationary. These machines are the cranes most frequently offered to contractors requiring a rental, as there is a lot to get wrong when operating these machines and special training is required for more advanced machines.
There is another form of crane that isn’t quite mobile but can be transported and erected anywhere, and that’s the tower crane. A familiar sight in large cities, these giants seem to pop up overnight yet remain installed for the months or years it takes to erect a skyscraper. You might be surprised to learn that many of these cranes take advantage of hydraulics.
Hydraulics at the heart
Indeed, if it moves and requires force and speed in a versatile package, you can bet that hydraulics is the first choice for motivation. Each of the above machines range from fully hydraulic (including drive systems) to partial usage. Every crane uses hydraulic cylinders for boom control outside of lattice boom cranes, which use wire rope hoists to lift or lower the boom.
Some of the largest cranes use a boom cylinder nearly large enough to walk through its barrel. With so much weight and safety on the line, you can bet boom hoist cylinders use sophisticated motion control systems (pilot-operated check valves need not apply). Although smaller machines may make do with a traditional cartridge valve, when heavy and expensive machinery is on the line, custom circuits are used to ensure safe lifting and lowering. Such circuits must offer zero leakage for load holding, no drop during boom-up function and no overshoot. It’s reasonable to expect the load control valve to use a half dozen or more valves to help prevent overload situations that could lead to tipping.
Such a counterbalance circuit will have multiple paths to prevent overloading. The primary counterbalance valve does the heavy lifting (pun intended), where it cannot be lowered except by receiving a pilot signal from the opposite work port of the directional valve. Rather than your traditional cartridge valve, you may also see slip-in logic elements to handle high flow. A separate relief ensures a load cannot begin to lift if the pressure at the cylinder’s cap port is too high. You may even see a reducing valve and orifice on the pilot input to prevent excessive flow and pressure on the control circuit.
Many cranes offer telescopic booms to extend many times their original length and reach higher than a fixed boom. Modern cranes use hydraulic cylinders to extend the boom, but some machines may still use cables. With cylinders, it’s easy to position the boom continuously between each section, where PO checks can safely lock them in place. Some systems use large bore cylinders with cable mechanisms similar to the chain and sheave mast operation of a forklift. When you see cranes with wide, square telescopic booms, it’s likely these are hiding such a cylinder and cable mechanism to improve effective stroke length.
Hydraulics ensures stable legs
Despite the hydraulic safety functions added, cranes still do tip, and nearly always for reasons outside of their hydraulic systems. The outriggers could be unevenly located, crosswinds might pull the load away from the crane’s center of gravity, or a dramatically oscillating load could lead to tipping.
Speaking of outriggers; these are another hydraulic actuator common to the crane. With high loads outside the machine’s center of gravity, providing a broad base is mandatory to achieve high load capacity. Outriggers, also called downriggers or stabilizing legs, move outward from the sides of the machine and then downward to the ground. Simple machines may operate fully manually but fully optioned cranes use hydraulic cylinders for both functions, which are controlled through the truck’s power system. Hydraulic outriggers can lift the crane entirely off the ground to ensure a stable platform with no central fulcrum to risk easy tipping.
Rather than push outward and then down, some outriggers pivot down and out to place the float pads onto the ground, while others move through one diagonal plane to press down a wide footprint.
Regardless of the method by which outriggers brace the ground, they’re all controlled by hydraulic valves. Directional valves could be lever or solenoid-operated and may operate simultaneously or individually. Using a single valve, the synchronized outriggers are best suited to flat pavement, while rough terrain is best suited to individual operation. In all cases, you can expect heavy use of pilot-operated check valves, which are less expensive than counterbalance valves and better suited to locking into a fixed position.
Erecting tower cranes
Unique amongst the cranes is the tower crane. Although these are sometimes manufactured with little hydraulic influence, this fluid power author believes the hydraulic machines are superior. Pretty much every tower crane uses hydraulics to elevate. The initial crane installation is aided by a truck-mounted crane, which hoists and places tower truss sections atop each other before finally lifting the slewing platform into place. However, this initial installation height is limited by the capacity of the helper crane, so subsequent sections must be added using hydraulics and the crane itself.
A structure called the climbing frame is installed at the bottom of the tower and then lifted to the upper-most truss section. Using hydraulic cylinders (usually referred to as “jacks” in crane nomenclature), the slewing platform is lifted to the height of the next truss section. The crane hoists the next section, and the trolly brings it to the face of the climbing frame. Now, with some manual labor, mechanics bolt the new section into place, and subsequent sections are added if required.
The luffing-style jib crane can tilt its mast to clear condos and towers in congested urban jungles, and although cables are often used, the final boss cranes use hydraulics. The use of a hydraulic cylinder for the luffing function provides a fast and reliable modification to the tower crane that reduces the complexity related to cable-only systems.
A hydraulic luffing jib crane only uses cables for the trolly and hook, which are functions also controlled by hydraulics. Even the slew motor is hydraulic, which is used to rotate the platform. And rather than a separate (sometimes portable) hydraulic pump for the jacking function, hydraulic quick couplers from the hydraulic system easily attach to the climbing frame for rapid lifting. However, secondary hydraulic power units may also be used.
When installed, the hydraulic jib crane looks like a modern fixed crane, as there is no jib tie mast to support the cables. This compact design also makes for improved ease of transportation and installation, as fewer components require assembly. Hydraulic luffing jib cranes have been in use for over fifty years, and for all their benefits, it’s surprising they’re not more popular.
Nevertheless, hydraulics as a whole is the most popular technology for the crane market. Its combination of power, versatility and reliability make it the first choice for most crane systems. You’re not likely to see rental yards free from hydraulically powered mobile cranes any time soon.
Filed Under: Components Oil Coolers, Cylinders & Actuators, Engineering Basics, Featured, Mobile Hydraulic Tips, Valves & Manifolds
