Contents
Overview
The parts of a tower crane and what each one does
The main parts of a tower crane start at the ground, with a base and a mast built up from lattice sections. On top, the slewing unit turns the jib and counterjib. The jib carries the trolley, with the hook block hanging below it on the hoist rope. The counterjib carries the counterweight, and limiters guard against overloading.
A tall crane is tied to the building beside it, and cranes that share a site are kept apart by planning backed by anti-collision systems. This guide covers tower cranes built from parts. Self-erecting tower cranes fold up for the road, as our guide What Is a Tower Crane? explains, and so do mobile tower cranes.
In short
- Under CPA guidance, a duty board on the tower shows the crane’s jib length, height under hook, ballast weights and its capacity at minimum and maximum radius.
- Capacity falls as the trolley runs out along the jib. On Liebherr’s 220 EC-B 12 flat top, the 12,000 kg maximum drops to 2,050 kg with the load out at the 68 m maximum radius.
- CPA guidance keeps at least 0.5 m clear all round a tower crane’s tower, so the tower has room to deflect.
- With the crane out of service, the loads on its base can be far higher than in use, CPA guidance warns.
Tower cranes are among the cranes our crane hire and contract lifting service supplies.
Sources
Where this guidance comes from
The general points on this page come from the Construction Plant-hire Association‘s Tower Crane Interest Group, whose guidance documents and technical information notes are linked where each point is made, and from HSE. The product standard for tower cranes, BS EN 14439, and the British Standard code of practice for using them, BS 7121-5, are described in our own words through our guides to them.
Every crane figure on this page belongs to Liebherr’s 220 EC-B 12 flat top, as its product page lists them, and holds for that crane alone. Other tower cranes differ in size, and makers do not always use the same name for a part. Anyone planning a real lift should work from the maker’s data and instructions for the crane on site, with the lift planned by a competent person.
At a glance
The main parts of a tower crane at a glance
The table starts at the ground and works up the crane, ending with its safety systems.
| Part | What it does |
|---|---|
| Base | Carries the crane into the ground or the structure below: a concrete foundation, a cross-shaped frame held down by ballast, or a steel grillage. |
| Mast sections | Lattice steel sections fixed one on top of another to make the tower. |
| Ties | Steel collars and legs that hold a tall tower to a structure beside it. |
| Climbing frame | A frame round the mast that jacks up the top of the crane so a new mast section can go in. |
| Slewing unit and slew ring | The section on top of the mast carrying the slew ring, the large bearing that lets the top of the crane turn, and the motors that turn it. |
| Cab | Where the operator drives the crane, high up beside the slewing unit. |
| Jib | The working arm. On a flat top or hammerhead crane the load runs in and out on a trolley; a luffing jib is raised and lowered instead. |
| Counterjib | The shorter arm behind the mast, which carries the counterweight. |
| Counterweight | Ballast blocks on the counterjib that balance the jib and the load. |
| Trolley | Runs along a level jib, carrying the hook in towards the mast or out to the jib end. |
| Hook block and hoist rope | The hook block hangs on the hoist rope, which a winch winds in and out to raise and lower the load. |
| Rated capacity indicator and limiter | Show the operator how close the crane is to its capacity, and stop movements that would overload it. |
| Anti-collision and zoning systems | Slow and stop the crane before it reaches a neighbouring crane, or keep its hook out of an area it must not enter. |
Standing
The base, mast, ties and climbing frame
The base
The base takes the crane’s weight, and the forces trying to tip it over, into the ground or the structure below. The CPA’s guide to top-slewing tower cranes (TCIG 0701) names four kinds: a concrete foundation with anchors cast into it, a static ballasted cruciform (a cross-shaped steel frame held down by ballast blocks), the same kind of frame on wheels running along rails, and a steel grillage fixed to the permanent structure. The same guide warns that the loads on the base with the crane out of service can be well above those in service.
Bases and ties count as temporary works, structures designed to support the crane only while it is on site. How they are designed and checked is covered in How Are Tower Cranes Erected and Dismantled?
The mast and its sections
The mast, also called the tower, is built from lattice steel sections fixed one on top of another, and its height is set by how many sections go in. The sections arrive on lorries, and a mobile crane lifts them into place on the base. TCIG 0701 says the tower must always have at least 0.5 m of clearance all round from any structure beside it, to allow for the tower deflecting.
Ties to the building
A tower can stand on its base alone only up to the free-standing height the maker gives for that mast and base. Above that it is tied to a structure beside it, such as the building going up, with steel collars round the mast and legs running across to the structure, as the CPA’s guide to climbing (TCIG 1101) describes. TCIG 0701 adds that an access route from a building to the crane must never be fixed to both the tower and the building, as that stops the tower deflecting and can bring the access down.
The climbing frame
A climbing frame fitted round the mast raises the top of the crane on hydraulic cylinders so a new mast section can be slid in underneath, and How Are Tower Cranes Erected and Dismantled? goes through climbing step by step.
At the top
The slewing unit, cab, counterjib and counterweight
Slewing unit and slew ring
Slewing means turning the top of the crane, so the jib swings the load round the site. The slewing unit sits on top of the mast and holds the slew ring, the large bearing the top of the crane turns on, along with the slewing gear that drives it. On its EC-B flat tops, Liebherr builds the slewing gear and the support for the slewing ring into one compact head with the crane’s central switchgear (its main electrical controls), and lists slewing gear of two 7.5 kW drives for the 220 EC-B 12.
A lift’s radius is the horizontal distance from the centre of the slew ring to the load’s centre of gravity, at the furthest point the load reaches, as our guide What size crane do I need? defines it. On a tower crane the centre of the slew ring sits over the middle of the mast.
When the crane is out of service, CPA guidance (TIN 038) has it left free to slew, so the wind can turn the jib to point downwind and present the least area to it. HSE’s safety bulletin on slew brakes followed a jib collapse in high wind, and asks for the slew brakes and their release gear to be kept in good working order.
The cab
Larger tower cranes are driven from a cab at the top of the tower, fitted at the slewing unit, which gives the operator a view over the site. Smaller ones, including self-erecting models, can be worked by remote control from the ground. TCIG 0701 asks for an anemometer, which measures wind speed, with its readout in clear view of the operator. For planning ahead, our wind check tool shows the Met Office forecast for a site and lets you adjust the gusts to the height of the lift.
Counterjib and counterweight
The counterjib is the shorter arm pointing the opposite way to the jib. It carries the counterweight, the ballast blocks that balance the jib and the load. TCIG 0701 notes that these blocks are often precast reinforced concrete, and has each one checked for damage, and its lifting points checked, before it is lifted into place.
A crane on a ballasted base carries two lots of ballast: the counter ballast on the counterjib and the base ballast that holds the base down. The duty board lists the weight of each.
Lifting
The jib, trolley, hook block and hoist rope
Hammerhead, flat top and luffing jibs
The jib is the long arm that carries the load. A hammerhead crane has a tower head standing above the jib, with tie bars between them. A flat top has no tower head or tie bars above the jib. Both keep the jib level, with the load on a trolley, and CPA guidance calls this kind of crane a saddle jib crane.
A luffing jib is raised and lowered to change the radius, and the load hangs from the jib tip. Out of service, TIN 038 has a luffing jib set at the minimum radius the maker gives for leaving the crane. Which type of jib suits which site is covered in What Is a Tower Crane?
The trolley
The trolley runs along the jib, driven by its own trolley travel gear, and the hook hangs below it. Moving the trolley out along the jib increases the radius, and the crane can lift less the further out the load goes. Liebherr gives the 220 EC-B 12 a maximum capacity of 12,000 kg and a maximum radius of 68 m, and with the load out at that 68 m radius it can lift 2,050 kg. The duty board shows the capacity at the crane’s minimum and maximum radius as rigged.
The hook block and hoist rope
The hook block hangs from the hoist rope, a wire rope wound on and off the drum of the hoist winch to raise and lower the load. CPA guidance (TIN 038) has the hook left empty when the crane goes out of service, raised to the position the maker’s manual gives. Our guide What is a LOLER inspection? covers how lifting equipment is inspected and thoroughly examined.
Safety systems
Capacity limiters and anti-collision systems
Rated capacity indicator and limiter
BS EN 14439, the product standard makers build tower cranes to, has requirements for devices that limit the crane’s loads and motions and show the operator what it is doing, as our BS EN 14439 guide explains. The rated capacity indicator shows the operator how near the load is to what the crane can lift at that radius, and the limiter stops the crane being overloaded. After the crane is put up, TCIG 0701 has both tested before it is handed over.
Anti-collision systems
CPA guidance (TIN 010) says an anti-collision system should be fitted where the risk assessment finds that cranes could hit each other’s structure or hoist rope. As two cranes close on each other, the system slows the movements of both, then stops them at a set buffer zone, a distance kept between the cranes. A specialist technician sets the system up on site. It is then checked each day and each week before work, and the operator signs for the checks.
TIN 010 is clear that these systems are a secondary aid to planning and communication. The crane coordinator, the person who plans the order of work between cranes sharing a site, keeps the cranes’ work apart in the first place, and BS 7121-5 sets out that role. Where several tower cranes share a site, TCIG 0701 calls an anti-collision radio system essential: a radio in each cab, on a frequency kept for the cranes, so the operators can talk freely.
Zoning systems
A zoning system slows and restricts the crane so its load, or parts of the crane, cannot enter an area they must not pass over. TCIG 0701 gives a railway line as an example of such an area.
The duty board
Under the CPA’s note on duty boards (TIN 007), a board fixed to the face of the tower, near its foot, sets out the crane’s make and model with its serial number, the weights of its base and counter ballast, its height under hook (the highest the hook can reach above the ground), its jib length, and its capacity at minimum and maximum radius, with the owner’s emergency contact. The board carries the site designation, so its figures are for the crane as rigged there.
From our lifts
We read the duty board against the lift plan: the jib length, the height under hook and the capacity at the radius each load needs. Where other cranes share the site, we check the anti-collision system has been set up and its checks signed for. Then we confirm the operator and the slinger signallers are on the same radio channel.
Send us your heaviest loads with where each one is picked up and where it lands. That sets the jib length and the capacity the crane needs at the jib end. Mark on the site plan anything the jib could swing over, such as roads, railways and neighbouring buildings, and tell us about any other cranes planned for the site.
Questions
Common questions
What are the parts of a tower crane called?
From the ground up: the base, the mast or tower, the slewing unit with its slew ring, and the cab. At the top, the jib reaches out over the site and the counterjib points back, carrying the counterweight. The hook hangs from a trolley that runs along a level jib.
What is the counterjib on a tower crane?
The counterjib is the arm that points away from the jib, loaded with ballast so the crane stays in balance when the jib carries a load. CPA guidance has the weight of this counter ballast shown on the crane’s duty board, alongside the ballast holding down the base where the crane stands on a ballasted frame.
What is the trolley on a tower crane?
The trolley is the carriage that runs in and out along a level jib, with the hook block hanging below it. Moving it changes the radius of the lift, and what the crane can lift drops as the trolley heads towards the jib end. The duty board gives the crane’s capacity at its minimum and maximum radius.
How is a tower crane attached to a building?
With ties. Each tie is a steel collar clamped round the mast, with struts running from it to the structure beside it. A tower needs them once it is taller than the maker allows it to stand alone. How ties go on as the crane climbs is covered in How Are Tower Cranes Erected and Dismantled?
How do tower cranes avoid hitting each other?
By planning first. A crane coordinator sets the order of work between cranes sharing a site, and the operators keep in radio contact. Anti-collision systems then back this up, slowing the cranes and then stopping them before they meet. CPA guidance says these systems are a secondary aid, never the only safeguard.
Why do tower cranes turn in the wind?
Because they are left free to slew when out of service. CPA guidance has the crane left so the wind can swing the jib round to point downwind, where it presents the least area to the wind. Cranes unable to turn this way have suffered serious damage and, in some cases, collapse, the same guidance notes.
Who checks the parts of a tower crane?
A competent person. In Great Britain, regulation 9 of LOLER requires a tower crane to be thoroughly examined after it is put up and before first use. While it is exposed to conditions causing dangerous deterioration, it is examined at least every 12 months, or under an examination scheme, and after exceptional circumstances. CPA guidance adds daily checks.
Get help
Get help with a tower crane
Call us on 0800 008 6096 or email office@synergy-lifting.co.uk. We supply tower cranes on contract lifts, with erection and dismantling arranged as part of the job. Send us a site plan with your heaviest load and the furthest radius it has to reach, and tell us how long you need the crane.
Find out more about our crane hire and contract lifting service. For mobile cranes, our guide Parts of a Mobile Crane Explained goes through each part.
Related
Find out more
Get a quote
Ready for a quote?
Send us the details and we will come back with a quote, usually the same day for a desktop survey.
Or call 0800 008 6096
