To choose crane capacity, span and lift height, start from your heaviest real load, the distance between your crane rails and the highest point the hook must reach. These three numbers decide the girder, the hoist, the motors and the building clearances, so they must be fixed before any drawing starts.
Many crane problems start with a wrong number at the enquiry stage. A crane that is too small is unsafe. A crane that is too big wastes steel, power and floor height. This guide explains each number in simple words, shows how to measure it on site and lists the mistakes we see most often in plants across Rajasthan.
Why crane capacity, span and lift height matter together
An EOT crane is one connected machine. If you raise the capacity, the girder becomes deeper and the hoist becomes larger. If you raise the span, the girder must be stronger to carry the same load without too much deflection. If you raise the lift height, the rope drum becomes longer and the hook needs more clear height under the roof.
So you cannot fix one number and guess the other two. Measure all three together, write them on one sheet and share that sheet with every crane maker you speak to. This makes quotations easy to compare and avoids changes after the order.
Step 1: Fix the safe working load (capacity)
Capacity, also called safe working load or SWL, is the maximum load the crane is designed to lift in normal use. It is marked in tonnes on the girder.
Follow these simple steps:
- List the heaviest item you lift today, such as a die, a coil, a motor or a fabricated job.
- Add the weight of slings, shackles, spreader beams, magnets or grabs that hang below the hook. These count as part of the load.
- Think about the next five to ten years. If heavier jobs are planned, choose the next standard capacity now. Upgrading a crane later costs far more than buying the right size first.
- Round up to a standard capacity such as 2, 3, 5, 7.5, 10, 15, 20 or 25 tonnes. Standard sizes keep spares simple.
Do not size the crane only on average loads. The heaviest lift decides the capacity. How often you lift near that weight decides the duty class, which is a separate choice. Our EOT crane duty class guide explains duty classes M3 to M8 in simple words.
Step 2: Measure the span correctly
Span is the distance from the centre of one crane rail to the centre of the other rail. It is not the inside width of the shed and it is not the column-to-column distance.
How to find it on site:
- If gantry girders and rails are already fixed, measure rail centre to rail centre at three or more points along the bay. Note the biggest and smallest value.
- If the building is new, start from the column centre distance shown on the civil drawing, then subtract the space taken by the gantry girder, the rail and the end carriage clearance. Your crane maker will confirm this from the drawing.
- Check that the rails are parallel. A difference of more than a few millimetres along the length can cause wheel wear and skewing.
Span has a strong effect on girder choice. For short and medium spans with light loads, a single girder crane is often enough. For longer spans or heavier loads, a double girder crane is usually the safer and more stable choice. See our single vs double girder EOT crane comparison for how to decide.
Step 3: Decide the lift height and hook height
Lift height, also called height of lift, is the vertical distance the hook travels from its bottom point to its top point. Hook height is the highest point the hook can reach above the finished floor.
To fix lift height, ask two questions. How high must the load go to clear machines, racks, pits or truck beds? And how low must the hook go, for example into a pit or onto a trailer? The difference between these two points, plus the height of the load and slings, is the lift height you need.
Remember that the crane itself also needs room. Above the hook there is the hook block, the hoist, the girder and a safety gap below the roof truss. If the rail level is fixed, a taller lift height may need a different hoist arrangement. If the building is new, plan the rail level after you know the lift height, not before.
Step 4: Check end approach and building clearances
End approach, also called hook approach, is how close the hook can reach to the end walls and to the side rails. A crane with poor hook approach leaves unused floor space at both ends of the bay.
Also check these clearances on site:
- Clear height from the top of the rail to the bottom of the roof truss, lights or pipes.
- Side clearance from the end carriage to the building columns.
- Space for the down-shop lead (power supply) or festoon cable along the runway.
- Space for a safe maintenance platform where needed.
Quick reference table
| Parameter | What it means | How to get the number |
|---|---|---|
| Capacity (SWL) | Heaviest load the crane lifts in normal use | Heaviest item + slings and attachments + future growth, rounded up to a standard size |
| Span | Rail centre to rail centre | Measure on site at several points, or take from the civil drawing |
| Lift height | Total vertical travel of the hook | Top hook point minus bottom hook point needed |
| Hook height | Highest hook point above floor | Height needed to clear the tallest obstacle with the load and slings |
| Long travel length | Length of the crane runway | Measure the full bay length the crane must cover |
| Duty class | How hard and how often the crane works | Number of lifts per hour, share of near-full loads, hours per day |
Common mistakes to avoid
- Giving column distance instead of rail centre distance as the span.
- Forgetting the weight of a magnet, grab or spreader beam.
- Choosing lift height from the roof height alone, without checking the space the crane itself needs.
- Ordering a crane before the rail level and gantry girder are confirmed.
- Sizing only for today and then needing a new crane in three years.
A good crane maker will visit the site, check these numbers with you and design the crane to IS 3177 and IS 807. At handover, every crane we build is load-tested at 125% of its rated capacity. Our guide to the 125% crane load test explains what this test proves.
What to send with your enquiry
Send one simple sheet with: capacity, span, lift height, long travel length, the type of loads, the number of lifts per shift, indoor or outdoor use, power supply available and any building drawing you have. If you are unsure about the hoist type, read our wire rope hoist vs chain hoist guide first.
FAQs
How do I calculate the capacity of an EOT crane?
Take the heaviest load you will lift, add slings, hook attachments and spreaders, allow for future heavier jobs and round up to the next standard capacity. Do not use average loads.
Is crane span the same as building width?
No. Span is measured from the centre of one crane rail to the centre of the other. It is always less than the building width and less than the column centre distance.
What is the difference between lift height and hook height?
Lift height is the total distance the hook travels up and down. Hook height is the highest point the hook reaches above the floor. A pit below floor level adds to lift height but not to hook height.
Can I increase the capacity of my crane later?
Sometimes, but it needs a full design check of the girder, end carriages, wheels, gantry girder and building. It is usually wiser to choose the right capacity at the start.
Who should measure the span and lift height?
You can take first measurements, but the crane maker should confirm them during a site survey before the design is frozen.
Talk to our engineers
R K Engineering Works, VKI Area, Jaipur, has designed and built EOT cranes since 1998 and is certified to ISO 9001, 14001 and 45001. Our team serves plants across Rajasthan. See our EOT crane range or book a site survey: call +91 9799108009 or +91 9413341806, or email contact@rkengineeringworks.in.