What is that? I searched it but couldn't find.
Any specification or technical data if you have, I would appreciate.

Not usually, if it is a large quantity the company will have two or more employees to help load or unload just as a time saver

I've mounted an extra guage too.
That is the point, the user fault.
Because there are too many rolls to unload sometimes more than 2 hours, and store them on top of each other it causes exhaustion for the driver.
So the reason I asked is, is it common for such a mistakes after a timely repetitive and exhausting job(maybe in 200 hundred rolls 1 drops.)?
If yes, is there any special tools, instrument or equipment for preventing this happening?

Driver may be at fault , do you have a crush gauge installed on unit for clamp pressure ?
If so may need to up the pressure

The clamp is single, rolls dia. Are max. 1100 mm. The grippers reach the middle roll smoothly.
This has been happening from the first day , and we culprit the driver for the most of the time as we couldn't find any hyd. Malfunction.

No it is not usual , once it is clamped it should stay that way no matter how many rolls you unload
First is the clamp a single or double ?
How big around are the rolls ?
Are clamp grip pads going at least halfway
On rolls ?
Did this issue just start. ?
If it holds for several rolls then after a while of use start dropping the rolls I would look at the hyd safety locks
They may be bleeding off causing to
Leak down and losing grip
Or a cyl issue
One or both cyls may have a little bypass
That can cause an issue
I usually reseal cyls and replace safety locks
As a safety measure

Thank you for your message.
Let me explain it more, actually the hydraulic system works functionally good the components and clamp jaws also are good enough.
The rolls weight are mostly more than 900kg , less 2000kg so the diameter is considerably big.
This accident occurs when more than 50 rolls would be unloaded.
I want to know that is it usual everywhere?
Is there any extra instrument or equipment to decrease the risk of this damage?
Or still you think it is a hydraulically malfunction ? Despite working many hours after that crash!

And as Scott pointed out grippers worn etc

Most clamps have pressure locks that lock with hyd pressure when closing the clamp and are released with pressure from opening
These are in the cyl hyd lines they are there as a fail safe to insure what you are describing doesn't happen they do have to be replaced over time but if you have to replace make sure you replace with ones that have the same ratings as old the numbers and ratings are on them

The rolls should never drop unless thereis a problem with the clamp as far as l know l would check the hyd pressure to the clamp most manufacturers have a Max pressure that goes to the clamp also there is on most clamps holding valves in all cylinders if a line goes it will not open the clamp next it is possible that the packing are going bad and bypassing also check the wear pads pins and the gripes they could wear out and cause the roll to drop l also have seen that the bushing in the ends of the cylinders to the pins wear and when squeeze in does not hold correctly. good luck