There are plenty of reputable suppliers willing and able to offer practical help, but as with any industry there is always a danger of them failing to explain the basics, particularly to someone buying lifting equipment for the first time. To address this, over the next few issues, this column will try to provide some straightforward advice on the various types of equipment available to aid the newcomer in selecting the right equipment for a particular application.
In this article, the spotlight falls on slewing jib cranes, or swing jib cranes as they are often called. Although we call them a crane, they are actually only a structure to support a lifting machine, such as a hand chain block or electric hoist. The lifting machine may be obtained from the same supplier or sourced separately.
A slewing jib crane comprises a horizontal runway track (the jib arm) cantilevered out from a vertical ‘king post’ located in bearings. The king post and bearings allow the jib arm to slew. The lifting machine is usually mounted on a trolley which travels along the jib arm.
The combination of slewing and trolley movement means that the lifting machine can be placed to lift and move a load to virtually any position within the arc of slew. In other words, it allows a load to be moved in all three planes.
There are many variants of slewing jib cranes and the following sets out the main options to consider. First is the method of supporting the jib arm. The simplest is for the king post bearings to be attached directly to an existing structure such as a building column or wall. Inevitably a wall mounted crane will be limited to 180 degrees of slew, whilst column mounted units can sometimes rotate up to 270 degrees.
Clearly the structure it is attached to must be adequate for the purpose and I always recommend that it should be assessed by a qualified engineer and written approval obtained. Otherwise some of the important detail checks can be missed.
The jib manufacturer should provide the loadings and the attachment points. However, the values provided will be based on certain assumptions which may not be true in the particular application. Remember that the jib is cantilevered and the supporting structure has to resist the maximum moment which is generated by the combined weight of the load and lifting machine when at the end of the jib. Moreover an allowance must be made for the dynamic effects of lifting and lowering the load. If a hand chain block is used, the effort force on the hand chain must also be taken into account. Finally, the supporting structure must be adequate at all positions within the arc of slew.
All steel structures deflect under loading so, if the jib is fixed to a building column, when the column deflects, two things can happen. First the jib can tilt from the horizontal. If excessive, the trolley will tend to run away downhill – not good.
Secondly, if the king post tilts from the vertical, the jib will tend to slew into the downhill position. Again, not good. Most modern standard industrial buildings are portal frame designs with metal cladding. Except for very low capacity short radius jibs, such buildings are generally too flexible without considerable reinforcement. Remember also that there are other loads on the building which can cause it to deflect, particularly those arising from the weather.
An alternative to the wall or column mounted slewing jib is a free standing jib with its own specially designed column. In most cases these types need special foundations of considerable depth to withstand the overturning moment. Only the lowest capacity short radius cranes can be fixed directly to the floor and then only if the floor specification is accurately known and suitable and the condition is good.
The column of a free standing jib crane can be cast directly into the foundation and this is the cheapest option so far as initial cost is concerned. It also keeps the space required around the base of the column to a minimum. However, in the event that it has to be moved at a later date, it is not so attractive. The alternative is a bolted connection. This does however necessitate a base plate and fixings which adds to the initial cost and requires more space.
Like a column mounted jib, if the king post bearings are mounted on the side of the column, the arc of slew of a free standing jib can be limited to about 270 degrees. Another variant using a tubular column allows a full 360 degrees of slew. However if this type is used with an electric hoist, the power supply has to be via slip rings unless stops are incorporated to prevent a full rotation.
Returning to the jib arm, there are two basic design options, over-braced and under-braced. As the names imply, this relates to how the cantilevered arm is supported. An over-braced jib has a tension member from the top of the king post to a point about two thirds of the way down the jib arm. The advantages are a lighter structure and one which allows the lifting machine to get as near as possible to the supporting structure giving the maximum effective travel. It also facilitates the use of special track sections instead of a rolled beam.
The disadvantage is that more headroom is needed for a given height of lift. An under-braced jib has the cantilever support beneath the runway beam. This keeps the headroom required to the minimum but necessitates a heavier rolled beam and reduces the maximum effective travel.
In most applications, the jib will be slewed and the trolley travelled by hand. Some caution is required, particularly if the jib is mounted very high. Pushing on the load will cause it to swing away from the vertical centre line of the jib. The greater the distance between the load and the jib arm, the more accentuated the swing will become.
When the jib arm then moves it will be delayed and then tend to snatch, causing the load to swing back and forth. This could endanger the operator and place excessive loading on the structure. A powered travel trolley is a useful feature and power slewing is also an option on the free standing 360 degrees style of crane.
A final consideration is the classification or duty of the crane. The British Standard BS 7333 divides them into eight classes based on a combination of the state of loading and the maximum number of operating cycles of the crane. The state of loading is a measure of the normal load lifted compared to the maximum load. So, light duty is where the maximum load is rarely lifted whereas very heavy duty is where the normal load is close to the maximum.
Essentially the classification is to ensure a reasonable working life of the crane. However there is another practical aspect to this. Light duty cranes are generally more flexible than heavy duty cranes. If the application requires precision positioning of the load, a bouncy crane is not as user friendly as a stiffer crane. Selecting the most appropriate lifting machine is equally important and will be the subject of another article.
