Multi-layer spooling and why pull drops

A winch does not pull with the same force whatever the state of its drum. It pulls hardest when the rope is on the first layer, against the bare drum, and the pull falls steadily as the drum fills with more layers, while at the same time the line speed rises. This is not a fault or a sign of a tired winch; it is simple leverage, and it is one of the most important things to understand when sizing a winch, because the headline pull is usually the first layer figure and the layers actually worked on may give considerably less. This builds directly on our notes on line pull and drum capacity.

Why pull falls as the drum fills

The winch produces a fixed torque at the drum, and the pull on the rope is that torque divided by the radius at which the rope leaves the drum. On the first layer the rope is at the smallest radius, the bare drum, so the pull is highest. As layers build up, the rope leaves at a larger radius, so the same torque gives less pull, falling layer by layer as the drum fills. It is the same effect as a longer lever arm reducing the force at the end, and it means the pull on the outer layers can be noticeably less than the first layer figure quoted on the datasheet.

Why speed rises as the drum fills

The flip side of the falling pull is rising speed. At a fixed drum rotation, each turn of rope hauls in a length equal to the circumference at that layer's radius, and a larger radius means a longer circumference, so each turn brings in more rope. As the drum fills and the radius grows, the line speed rises even though the drum turns at the same rate. So pull and speed trade against each other across the layers: the first layer gives the most pull and the least speed, the outer layers the least pull and the most speed, the same energy delivered differently as the drum fills.

How much the pull drops

The size of the drop depends on how much the radius grows from the first layer to the outer one, which in turn depends on the drum and the rope. A drum that holds many layers of rope sees a large change in radius from bare drum to full, so the pull on the outer layers can be much less than on the first, while a drum holding only one or two layers sees little change and little drop. So the more layers a drum holds, the bigger the difference between the first layer pull and the outer layer pull, which is why a winch holding a long rope in many layers needs this drop kept firmly in mind.

The first layer figure on the datasheet

Datasheets almost always quote the line pull on the first layer, because it is the highest figure, the same point our note on reading a datasheet stresses. This is fair enough as a stated maximum, but it can mislead if the work is done on the outer layers, where the pull is lower. Reading a pull figure, the first question is which layer it refers to, and the next is what the pull will be on the layer the job actually uses. A winch chosen on its first layer pull alone may fall short on the outer layers where it really works, which is a common and avoidable disappointment.

Allowing for it when sizing

The honest way to size a winch is to work out the pull on the layer the job will actually use, not just the first layer figure. If the rope is mostly worked from the outer layers, the winch must be chosen so the pull there is still enough, which usually means a winch rated higher on the first layer than the bare requirement, so the outer layer pull meets the need. This is why the drum, the rope and the layers worked are considered together with the pull, so the winch delivers enough force where the job is done, rather than only on a first layer the work rarely uses.

Designing the drum around it

The pull drop can also be managed by the drum design. A drum sized so the job is worked on the inner layers, with the outer layers used only for stowing spare rope, keeps the working pull high. A larger barrel diameter reduces the proportional change in radius across the layers, lessening the drop. So the drum is not just sized to hold the rope but shaped to keep the working pull adequate across the layers in use, which ties into our note on drum and rope capacity. Designing the drum with the pull drop in mind is part of giving a winch enough force where it works.

Pull, speed and the duty

The trade between pull and speed across the layers can even be used. Where high pull is needed, the job is worked on the inner layers; where speed matters more than peak pull, the outer layers give it. So a winch with several layers offers a range of pull and speed across the drum, and the duty can be matched to the layers that suit it. But this only works if it is understood and planned, rather than discovered when a winch chosen on the first layer pull proves too weak on the outer layers. We size the winch and design the drum so the pull and speed suit the duty across the layers actually used.

Sizing for real pull with us

We size the winch and the drum so the pull is enough on the layers the job actually uses, not just the first layer figure. See the range in our winch catalogue, and read how the line pull, the drum capacity and reading a datasheet fit together. Tell us the pull you need and how much rope is out in use, and we will size a winch that delivers the pull on the working layers rather than only on the first.

Frequently asked questions

Why does a winch pull harder on the first layer?

Because the winch produces a fixed torque at the drum, and the pull is that torque divided by the radius at which the rope leaves. On the first layer the rope is at the smallest radius, the bare drum, so the pull is highest, and it falls as layers build up and the radius grows. It is simple leverage.

Why does the line speed rise as the drum fills?

Because each turn of the drum hauls in a length equal to the circumference at that layer's radius, and a larger radius means a longer circumference. As the drum fills and the radius grows, each turn brings in more rope, so the line speed rises even though the drum turns at the same rate.

How much does the pull drop across the layers?

It depends on how much the radius grows from the first layer to the outer, which depends on the drum and rope. A drum holding many layers sees a large change in radius and so a large drop in pull, while a drum holding one or two layers sees little change and little drop.

How do I allow for the pull drop when sizing a winch?

Work out the pull on the layer the job actually uses, not just the first layer figure. If the work is on the outer layers, choose a winch rated higher on the first layer so the outer layer pull still meets the need. The drum, rope and layers worked are sized together with the pull.