Components of Gantry Cranes
We shall examine the components of a typical gantry crane running on steel overhead rails mounted inside a building.
1. Steel Rail
These are manufactured in various lengths from cast iron having a bottom flange that is drilled for high tensile bolts and nuts used to fix the rails to the building’s main steel work. There are two methods of fitting the bolts.
a) Bolts inserted downwards with the nuts fitted to the bolts from underneath. This prevents the bolt falling to the ground should the nut slacken off. However there is no visible warning of a nut missing until it hits the floor (or your head)!
b) Bolts inserted upwards with the nut fitted from above. This method allows the operator to notice right away that the bolt is missing as the bolt hole will be visible. However some of these bolts are quite large and heavy. This combined with the height of the rails are also capable of inflicting injury should one fall and hit a worker.
2. Wheels and Drive Motor
The long-travel wheels are also cast having steel flanges and double ball bearings. Two of the wheels are driven by electric motors and two are normally free running. These are contained in the end carriages that are fitted to each side of the crane beam(s).
The electric drive motors may be mounted directly onto the driven wheels or mounted on the cross-beam and driving the wheels via a drive shaft.
It is very important to have these long-travel drive motors synchronized to stop and start at the same time as any time lag at all will result in the crane “crabbing" (the name given to the ‘Z’ action that the crane adopted on starting).
I was engineer at an aluminum smelter here in the North of Scotland for many years. The cast shop had four overhead gantry cranes of 50T & 20T capacity. The 50 tonners were used to transport molten aluminum in steel crucibles to the holding furnaces, prior to casting. One particular crane was prone to crabbing, which was a dangerous situation to be in when travelling up the shop with a crucible of molten metal hanging from the hook!
We tried numerous times to get the motors synchronized to start together at the same time. One quite weekend nightshift, I operated the crane and the electrician and fitter observed the drive motors as I stopped and started the travel. We suspected backlash in the drive gears and shafts, but eventually we traced the fault to the magnetic brake on one motor that was lagging ever so slightly behind the other one. Once we adjusted the clearance and greased the linkage, the crane worked satisfactorily.
Some years later the smelter closed and the cranes and the equipment were sold to a smelter in Dubai. I applied for an engineer’s post out there and was flown to the plant for final interview.
I was being shown around their cast shop and lo and behold; up above me was one of our old overhead cranes. I mentioned this to the plant manager and he said they were excellent cranes (Vaughn’s I think they were) except for their crabbing! I was able to tell him how to put this right. In the end I was offered the engineer’s job, but turned it down accepting an alternative position with a local oil and gas offshore construction company.
Anyway, this is one thing to look out for that can be very dangerous when lifting and transporting heavy loads; especially when maneuvering in tight locations.
3. Cross Travel Bogie/Trolley
The bogie is electrically driven and houses the main hoist. It is under-slung on a single beam or suspended between two beams in a double beam design. The electric drive motor enables the bogie to cross-travel along the beams.
The hoist is mounted on the cross travel bogie and consists of a steel drum around which the coils of wire rope are wound. It is electrically driven and has the normal magnetic brake that operates on raising or lowering the crane hook, whether loaded or unloaded.
5. Operator's Ca
Accessed by a set of vertical looped ladders, the cab is located at one end of the overhead beams and gives all-round vision to the crane operator. The long travel, cross travel, and hoist are normally hand operated from a control panel convenient to the operator’s seat, with an emergency long travel hydraulic foot brake on the floor of the cab.
A sketch showing the components follows.