A telescopic boom crane is a hydraulic crane with a straight boom made of steel sections that slide out of one another, like a telescope, to reach further. Mounted behind the cab of a truck, it is usually called a boom truck or a telescopic truck mounted crane. One hydraulic cylinder raises the boom, other cylinders push the sections out, and a winch raises the hook on a wire rope.
This guide explains how the boom, its cylinders and the telescoping system work, what happens to the lifting capacity as the boom grows longer, and where a telescopic crane is the better choice than a knuckle boom. For this guide we compiled the technical drawings of our twelve current telescopic truck mounted cranes, from 2 t to 20 t, and every crane figure below is taken from them. Achord Chan, Product Operations Director at BOB-LIFT, prepared it with our engineering team.
Key takeaways
- A telescopic boom crane lifts with a straight boom of nested sections; our current models have three to five sections and reach from 8.2 m to 22.3 m.
- The luffing cylinder sets the boom angle, while the telescopic cylinders set the boom length, and the two together decide the working radius.
- Capacity falls as the radius grows: our 10 t SQ10SA4 lifts 10,000 kg at 2.4 m but 1,050 kg at 15.5 m.
- At the same radius, a shorter boom at a lower angle usually lifts more than a longer boom at a steeper angle, so use the shortest boom that reaches.
- Choose a telescopic crane for long, straight reach and lifting on a hook; choose a knuckle boom when you need compact storage and placing loads over obstacles.
What is a telescopic boom crane?
A telescopic boom crane is a crane whose boom is a set of box-section steel tubes nested inside each other, extended and retracted by hydraulic cylinders. The outermost and largest section, pinned to the top of the column, is called the base boom or main boom; the others are the telescopic sections. When the boom is fully retracted, all the sections sit inside the base boom, and the crane is at its shortest and strongest.
On a truck, the crane is bolted to the chassis behind the cab and draws its power from the truck engine through a power take-off. When the work is finished, the boom is retracted and laid along the truck bed, and the truck drives away with its load. In North America this machine is called a boom truck; in most other markets it is a telescopic truck mounted crane, a straight boom crane or a stiff boom crane.
Among our truck mounted cranes, telescopic boom models run from 2 to 20 t and knuckle boom models from 1.6 to 50 t.
How does a telescopic boom crane work?
Every movement of a telescopic boom crane is hydraulic, and the oil follows one path: the truck’s engine drives a pump through the power take-off, the pump sends oil to the control valve, and the operator’s levers send that oil to four kinds of actuator. Our guide to truck mounted crane parts numbers each part on a real drawing; the movements are:
- Slewing. A hydraulic motor and gearbox turn the column on its slewing bearing; all our telescopic truck cranes slew through 360 degrees.
- Luffing. The luffing cylinder, between the column and the underside of the base boom, raises and lowers the boom; our telescopic cranes reach a maximum boom angle of 75 degrees.
- Telescoping. The telescopic cylinders inside the boom push the sections out and pull them back in, which changes the boom length.
- Hoisting. A hydraulic winch winds the wire rope that runs over the sheaves in the boom head down to the hook block, which raises and lowers the load.
The boom angle and the boom length together set the working radius, which is the horizontal distance from the slewing centre to the hook. Before any of this happens, the outriggers are extended and lowered so that the load goes into the ground; our article on outriggers and ground bearing pressure explains how to set them up.
How the telescopic boom cylinder works
The telescopic boom cylinder, also called the boom extension cylinder, is the hydraulic cylinder inside the boom that pushes the sections out. Because it lives inside the boom, oil often reaches it through tubes or hose reels inside the boom rather than through external hoses. Each cylinder that holds a load, including the luffing cylinder and the extension cylinders, is normally fitted with a load-holding valve, so the boom stays where it is if a hose fails or the lever is released.
Manufacturers use two main ways to move several sections with cylinders, and many cranes combine them:
- Sequential telescoping. The sections extend one after another, in a fixed order, each driven by its own cylinder or by one stage of a multi-stage cylinder. Sequence valves decide which section moves first.
- Synchronised telescoping. All sections extend together at the same rate. Typically one cylinder drives one section, and ropes or chains running over pulleys inside the boom pull the next sections out with it.
- Combined systems. Cylinders extend some sections in sequence, while ropes extend the last sections in step with them, which keeps the boom light while giving a long reach.
Whichever system a crane uses, the buyer sees its result in two places on the technical drawing: the number of boom sections and the boom lengths printed at the top of the load chart. Our 10 t SQ10SA4 has four sections, one main boom and three hydraulic extensions, and its load chart lists four boom lengths: 5.6 m, 9.02 m, 12.44 m and 15.87 m.
Boom length, boom angle and lifting capacity
The load chart on the technical drawing gives the rated load at each combination of boom length and boom angle. The chart below plots the full table for the SQ10SA4, so you can see how capacity changes as the boom grows longer and drops lower.
| Boom angle | 5.6 m boom | 9.02 m boom | 12.44 m boom | 15.87 m boom |
|---|---|---|---|---|
| 0° | 4,500 kg at 5.0 m | 2,200 kg at 9.0 m | 1,500 kg at 12.0 m | 1,050 kg at 15.5 m |
| 30° | 5,700 kg at 4.3 m | 2,700 kg at 7.6 m | 1,810 kg at 10.5 m | 1,300 kg at 13.5 m |
| 50° | 8,700 kg at 3.2 m | 3,600 kg at 5.7 m | 2,450 kg at 7.8 m | 1,850 kg at 10.0 m |
| 60° | 10,000 kg at 2.4 m | 4,300 kg at 4.4 m | 3,000 kg at 6.1 m | 2,170 kg at 7.8 m |
| 75° | Not listed | 7,000 kg at 2.3 m | 5,100 kg at 3.2 m | 3,400 kg at 4.2 m |
Three practical lessons come out of this table. First, capacity depends on radius much more than on boom length: at the same 7.8 m radius, the 12.44 m boom at 50 degrees is rated for 2,450 kg, while the 15.87 m boom at 60 degrees is rated for 2,170 kg. Second, extending the boom has a cost even when the radius stays the same, because the extra sections add their own weight to the boom. Third, the full rating of 10,000 kg applies only at 2.4 m with the shortest boom, so the headline tonnage tells you little about the load you can place at the back of the truck bed or over a wall. For a step-by-step method, see our guide on how to read a crane load chart.
Telescopic boom crane sizes and sections
The number of sections grows with the size of the crane, because a longer boom needs more sections to retract into a length that fits on the truck. The table lists four of our twelve telescopic models; the full range, with weights and installation space, is in our article on boom truck sizes.
| Crane | Boom sections | Max. radius | Load at max. radius | Oil flow and pressure |
|---|---|---|---|---|
| 2 t telescopic | 3 (main + 2) | 8.2 m | 250 kg | 20 L/min, 16 MPa |
| 5 t SQ5SA3 | 3 (main + 2) | 8.68 m | 1,100 kg | 32 L/min, 20 MPa |
| 10 t SQ10SA4 | 4 (main + 3) | 15.5 m | 1,050 kg | 63 L/min, 26 MPa |
| 16 t telescopic | 5 (main + 4) | 22.3 m | 1,050 kg | 80 L/min, 30 MPa |
Two cranes with the same rated capacity can be built very differently. Our two 5 t models show it clearly: the three-section SQ5SA3 reaches 8.68 m and lifts 1,100 kg there, while the four-section 5 t model reaches 11.79 m and lifts 600 kg at that radius. The extra section buys about 3 m of reach in exchange for roughly half the load at the end of the boom.
What is a telescopic boom crane used for?
Telescopic truck cranes suit jobs where loads hang from a hook and have to travel a long, straight distance from the truck. Typical examples include unloading steel, pipes and precast concrete, setting machinery and transformers, lifting building materials onto a roof or over a fence, and loading containers or generators. Because the hook hangs on a rope, the crane can also lower loads below ground level into trenches and pits, as long as there is enough rope on the winch drum.
Telescopic boom vs knuckle boom
The main alternative is the knuckle boom crane, whose boom folds at a joint. Neither type is better in general; each suits a different kind of work. Our detailed comparison of knuckle boom and telescopic boom truck cranes sets five pairs of real models side by side.
| Question | Telescopic boom crane | Knuckle boom crane |
|---|---|---|
| Boom shape | Straight, nested sections | Inner and outer boom joined at a knuckle |
| Lifting point | Hook on a wire rope from a winch | Hook at the boom tip; winch optional |
| Stowed on the truck | Boom lies along the bed and takes deck length | Folds behind the cab and leaves the deck free |
| Best at | Long, straight reach and lifting below ground level | Precise placing, reaching over obstacles, attachments such as grabs |
In the United States, a telescopic boom truck used in construction falls under OSHA’s crane standard, 29 CFR 1926.1400, which sets the scope of the rules on its assembly, inspection and operator qualification. The design and inspection of mobile cranes, including truck-mounted telescopic cranes, is covered by ASME B30.5. Check the rules that apply where your crane will work before you order it.
Frequently asked questions
Is a boom truck the same as a telescopic crane?
Usually yes. In North America, boom truck means a truck with a telescopic crane mounted behind the cab, and the same machine is called a telescopic truck mounted crane or straight boom crane in most other markets.
What is the boom cylinder on a crane?
Boom cylinder can mean either of two cylinders. The luffing cylinder, also called the lift cylinder, raises and lowers the boom, while the telescopic or extension cylinder inside the boom pushes the sections out and pulls them in.
How many sections does a telescopic boom have?
On truck mounted cranes, usually three to five, counting the main boom. Our 2 t and SQ5SA3 have three sections, the 10 t SQ10SA4 has four, and our 8 t, 14 t, 16 t and 20 t models have five.
Can a telescopic crane lift at full reach?
Yes, but only a small part of its rated load. Our 10 t SQ10SA4 is rated for 1,050 kg at 15.5 m and our 20 t SQ20SA5 for 1,500 kg at 22 m, so always check the load chart at your actual radius.
What is the maximum boom angle of a telescopic truck crane?
Our telescopic truck cranes reach a maximum boom angle of 75 degrees, which gives the shortest radius and the highest rated load for each boom length.
Choose the right telescopic boom crane
Tell us your heaviest load, the longest radius you need to reach and the truck you plan to use, and we will propose a telescopic or knuckle boom model with its technical drawing and load chart, so you can check the figures before you decide. Contact us for a quotation.
