If the gantry and crane are part of a completely new project then, whether outdoor or indoor, there is a large degree of design freedom. Compared to the installation of a gantry into an existing building, there are few, if any, constraints. Nevertheless there are some pitfalls to avoid, particularly where the budget is tight and there is a temptation to make false economies.
It is well worth applying a little foresight and investing in some future proofing features. I will come to these later.
Alongside new installations, another common scenario is the purchase of a second hand gantry as part of the acquisition of an existing building or site. In this situation, you will discover just how much foresight the original buyer exercised.
Whatever the variant of overhead travelling crane to be used, there are basically two types of gantry: the freestanding design, and those supported by a structure such as a building. Any outdoor gantry is usually freestanding, but it is also an economical solution for some indoor applications, particularly where the crane is not required to service the entire volume of the building.
It can be particularly appropriate for low-capacity light duty cranes of the type built from a kit of standard parts and utilising proprietary track sections. If the gantry is supported by the building, it may have been designed to carry the gantry from the outset or it may have been adapted at a later date.
As well as the two basic types, there are also two variants of gantry. One is for an underslung crane, the other for a top running crane.
The requirements for both types and variants have a lot in common, and perhaps the most obvious is adequate strength. However this encompasses not just the vertical loads imposed but also the transverse and longitudinal loads. To travel a load the crane must accelerate and decelerate it and that transmits forces sideways and along the length of the gantry. These in turn are transmitted to the supporting structure.
The forces arise not just from the payload but also the weight of crane hoist or crab and the crane bridge (depending on direction of travel). Moreover there is the possibility of shock loading arising from accidental collision into the travel end stops at speed.
I recollect the case of a new building where, during construction, it was realised that an area needed to be served by an overhead crane. It was too late to make the gantry integral with the building frame so a semi-independent structure was added. This took the vertical loads imposed by the crane but was tied into the building frame for lateral stability.
The building frame was in-filled with brickwork and the structural engineer clearly thought it would be adequate. However the day soon came when the crane hit the long travel end stops and the entire 80ft wide brick gable end shattered from top to bottom. An expensive error, to say the least.
There are now several very effective technical solutions for avoiding collisions between the crane and the gantry end stops and also between cranes on the same gantry. The modern approach to safety is to eliminate the hazard wherever possible by designing it out or guarding against it. Relying on operator skill alone should be low down in the hierarchy of measures. However there are still many older installations in use which have little or no collision protection and anyone buying or inheriting such an installation is advised to evaluate the risks it presents. Anti-collision equipment can often be retrofitted to existing cranes.
Irrespective of the anti-collision measures in place, the gantry and its supporting structure must be of adequate strength for the foreseeable loads which can be imposed. This includes the provision of adequate end stops or buffers which engage at the correct height with the crane end carriage so as to prevent the crane running off the end of the gantry.
Closely related to strength is stiffness, which was mentioned last month in the context of the crane bridge. Essentially, every structural element deflects under load and, whilst each element may be within acceptable limits, the cumulative effect may not be acceptable for two reasons.
First, it may result in excessive slope allowing the load to run away downhill or making travel uphill more difficult. Deflection affects not just the individual gantry beam but also the slope of the crane bridge between the two gantry beams and hence the cross travel motion. Secondly, an excessively flexible structure can make it difficult to accurately position a load. A bouncy structure is not user-friendly.
After strength and stiffness an important feature is adjustability. Crane gantry beams must be straight, level, level to each other and parallel within quite small tolerances.
If they are not, then the effect on the crane can vary from causing undue wear and tear to the serious danger of derailment. These tolerances are smaller than can generally be expected of the supporting structure so a means of adjustment is essential. Moreover, with time, supporting structures can move due to settlement and accidental damage and this is where some foresight is invaluable.
It is not unusual to find that, due to uneven floor loadings or heavy vehicle traffic at one side, a building has settled into a banana shape and a straight, level gantry of the required span is no longer possible.
Equally, industrial buildings are often added to or modified and this can create local distortion. Therefore as well as providing adjustment for the initial alignment, it is important to ensure that there is sufficient scope for future re-alignment.
Also avoid, if possible, being too tight with the end clearance between the gantry beam and the building structure. It is tempting to minimise this to get the best hook approach but it can limit the scope for re-alignment and also limit the options if, at some future period, a new crane is required. The various makes and models available on the market have differing clearance dimensions.
At some point you may also require an additional crane or one of higher capacity. This brings us to the crystal ball stage. Whilst it is entirely foreseeable that the gantry will need to be initially aligned and at some time re-aligned, your future crane requirements are perhaps more speculative. If however you are in a position to forecast with any accuracy, it is worth remembering that the additional cost of extra capacity at the initial stage will be far less than the cost of modification later.
If you are acquiring a building or site with an existing gantry, it is advisable to have it professionally surveyed by a specialist and also have the design checked as it may not be entirely clear what working load it is rated for. The original design calculations were probably made for a particular crane and would take account of the maximum wheel loadings, the distance between the end carriage wheel centres and the duty rating of the crane.
Generally modern cranes have a lower self weight than older ones, resulting in lower wheel loadings, but may also have less distance between wheel centres. The current application may require a different duty rating to that allowed for in the original design.
Therefore it does not necessarily follow that the gantry is suitable for a modern crane of the same safe working load. However, on the positive side, if the new crane is only for light duty, a significant capacity up-rating is often found to be possible.
If the gantry is supported by a building and there is evidence that it is an addition rather than part of the original design, particular attention should be paid to checking that the combined structure is adequate for all the loads imposed.
A point to remember is that the span of a portal frame building can vary according to wind load, snow load, loading on mezzanine floors etc. The variation can be more than the allowable tolerances for the gantry and may require additional ties to stiffen the structure. With regard to condition, over time the gantry rail or, in the case of an underslung crane, the whole track, will wear and ultimately will have to be replaced. If the alignment has been poor it may have already caused excessive and uneven wear.
Replacing the worn rail or track can be expensive but failing to do so will cause premature wear and tear on the new crane and may also be unsafe. As well as the running surfaces, the survey should include the condition of the supporting structure. It may be suffering from corrosion, fatigue, cracked welds, loose bolts or rivets and collision damage.
Once the fundamentals of design and condition have been checked and found to be satisfactory, the alignment should be checked and adjusted as necessary.
Finally, your electric overhead travelling crane will need a power supply to it and this is usually attached to one of the gantry beams. On an existing gantry, check whether it is safe and whether it is adequate. It is not too long ago that the usual down-shop power supply was by bare copper conductors. I certainly remember them and also what happened when one broke. There are better sources of excitement in life.
Older three phase conductor systems often only have three conductors, the neutral being earthed through the gantry.
However any out-of-balance between the phases will result in a current flow through the neutral and sometimes the path of least resistance is not the gantry but down the wire suspension cable to the push button control and thence the operator’s arm to earth. Guess how I know.
That should not occur with a modern well insulated control but a fourth conductor addresses the problem at source.
With regards to adequacy, the conductors need to have adequate capacity to power all crane motions simultaneously, taking account of the permitted variation in the supply voltage and the voltage drop arising from the length of the system. Of course, if there is more than one crane sharing the system, this must also be taken into account.
A customer of UK-based Street crane required the removal of two existing 5t overhead cranes and their gantries, from one hangar to another, as well as the provision of a new 3.2t crane for the new fabrication area street If the gantry and crane are part of a completely new project then, whether outdoor (as in this example) or indoor, there is a large degree of design freedom SCA overhead crane in fog Whatever the variant of overhead travelling crane to be used, there are basically two types of gantry: the free standing design, and those supported by a structure such as a building Double girder crane for industrial gas production An example of an underslung crane underslung LEEA chief executive Derrick Bailes LEEA chief executive Derrick Bailes
