Decoiler Design Calculations: Coil Car, Mandrel Drive, Expansion and Press Arm
Why the rails can matter more than the coil, why speed costs more than weight, and how to size every motor and cylinder on a decoiler, using one worked example you can follow and change.
A common shortcut in decoiler design is to take the motor from a similar machine, choose the next size up and add reduction. It often works, which is why it persists. When it does not, the problem appears at the trial run: a coil car that moves on one site and stalls on another, a mandrel drive that struggles with a full coil, or a design that gets heavier and more expensive with every project.
This guide follows one decoiler, module by module, with one set of numbers. Every result can be traced back to its inputs, the formulas are one click away, and four small calculators let you change the inputs yourself.
A decoiler is designed around its mandrel: it turns to pay off or rewind the strip, expands to grip the coil and contracts to release it. The coil car, frame, press arm and side guides exist to load, carry, hold or guide that coil. Size their motors and cylinders from the coil, the line and the site, not by copying the last machine and adding one size. The inputs that change the answer most are the payoff mode and its control, the line speed, and site conditions such as rail installation. Coil weight matters, but usually less than people expect.
01Anatomy
Everything on a decoiler is built around the mandrel
The mandrel does four things. Every other part of the decoiler exists to serve those four motions.
Heavy coils also need an outboard support at the free end of the cantilevered mandrel, and automatic centering or one-touch loading places the coil accurately on it. The sizing logic below applies to those parts in the same way.
The example we follow
One heavy coil used in every calculation, so the results can be compared. It is a design example, not the specification of a particular machine.
- Coil weight
- 15,000 kg
(33,070 lb) - Strip
- 1.0 ร 1,400 mm
(0.039 ร 55.1 in) - Coil ID / OD
- 508 / 1,400 mm
(20 / 55.1 in) - Line speed
- 30 m/min
(98 ft/min) - Material
- Steel, 7.85 g/cmยณ
- AC motors
- 1,400 rpm
- Gravity in force checks
- g โ 10 m/sยฒ
- Your own coil
- Coil weight and PIW calculator
02Coil car
The rails decide the coil car motor, not the coil
A coil car has two design questions. The static one, structure and lifting capacity, follows the coil weight and the loading method. The dynamic one, how much motor it takes to move the loaded car, is decided mostly by the track.
On perfectly level rails with perfectly rigid wheels, a loaded car on good bearings would need very little force to move, whatever it carries. Real rails are never exactly level or straight, and wheels and rails flatten slightly where they touch. The drive calculation therefore carries two correction terms: a rolling resistance ฮผ and a rail installation allowance k.
Where the drive force comes from in the worked example
The site decides the motor. Two-thirds of the calculated force comes from how the rails are installed, not from rolling resistance. That is why the same coil car can run at one plant and stall at another: the coil did not change, the track did.
Show the calculation
| Worked example | Value |
|---|---|
| Coil + car weight | 15,000 kg + 2,206 kg |
| Wheel diameter ยท reducer ยท sprockets | 100 mm ยท 1:90 ยท 25 / 30 |
| ฮผ ยท k ยท ฮท | 0.05 ยท 0.10 ยท 0.85 |
| Travel speed (at least 3 m/min required) | 4.07 m/min |
| Travel time over 2,720 mm (4,170 mm rail minus 1,450 mm wheel span) | 40 s |
| Drive force ยท calculated power ยท selected motor | 25.8 kN ยท 2.06 kW ยท 3 kW |
In practice: check rail level and straightness when the track is installed and keep the rails clean. For car alignment and maintenance, see the coil car setup and centerline alignment guide; for standard configurations, the decoiler with coil car.
03Mandrel drive
Mandrel drive: speed costs more than weight
Three inputs set the mandrel motor, in decreasing order of influence:
Payoff mode and controlDoes the motor drive the coil (active payoff), or does the next machine pull the strip off (passive payoff)? Top or bottom exit, fixed-speed or speed-controlled drive belong to the same decision.
Line speedThe speed the decoiler has to keep up with.
Coil weightIt matters, but less than the first two. The coil also gets lighter as it unwinds, so a motor sized only for the heaviest moment is oversized for most of the coil.
To handle a bigger job, which raises the mandrel motor more: doubling the coil weight, or doubling the line speed?
Show the answer
Doubling the speed. The power needed to accelerate a coil grows with the square of its rotating speed, so about four times, but only in proportion to its mass, so about twice.
Check the speed at the inner diameter
With a fixed drive ratio, the mandrel turns at the same speed from a full coil to an empty one, so the strip moves slowest at the end of the coil. In the example the mandrel turns at 21.4 rpm: that is 94.3 m/min of strip at the full 1,400 mm OD but 34.2 m/min at the 508 mm ID, still above the 30 m/min the line needs.
Checking the speed at the ID confirms that the decoiler keeps up to the last wrap.
Fixed speed or speed-controlled: same coil, two answers
A fixed-speed motor switched on and off by a loop sensor takes the full coil to full mandrel speed at every start: 21.4 rpm, or 94 m/min of strip, three times what the line needs. A variable-frequency drive (VFD) that follows the 30 m/min line only needs 6.8 rpm on the full coil. With 2 s to accelerate and 80% drive efficiency, that is the difference between 12.8 kW and 1.3 kW.
Show the calculation
| Worked example | Value |
|---|---|
| Coil mass ยท moment of inertia | 14,690 kg ยท 4,073 kgยทmยฒ |
| Mandrel speed (1,400 rpm motor, 1:30 reducer, 17 / 37 sprockets) | 21.4 rpm (2.25 rad/s) |
| Strip speed at the ID / at the full OD | 34.2 / 94.3 m/min |
| Strip length and run time at 30 m/min | 1,337 m ยท about 45 min |
| Acceleration power, fixed speed (t_a = 2 s, ฮท = 0.8) ยท selected motor | 12.8 kW ยท 15 kW |
| Acceleration power with a VFD at 30 m/min | 1.3 kW |
A VFD ramp also softens every start; see how a VFD soft start reduces coil scratches. For standard motorized models and their drive options, see the decoiler machine range.
04Mandrel expansion
Mandrel expansion: link or wedge, and the 8.5ยฐ rule
A wedge locks itself when its angle ฮฑ is smaller than the friction angle ฯ = arctan ฮผ. With a friction coefficient of 0.15, ฯ = 8.5ยฐ, and the example uses a 14ยฐ wedge. Above the friction angle, the segments are free to retract when the pressure is released, so the coil can be unloaded; in return, the hydraulic circuit has to hold the expansion pressure while the coil runs. A smaller angle gives more radial force for the same cylinder, but less margin against sticking.
Standard and medium coils. A mature mechanism: a standard cylinder bore is normally sufficient.
Heavy coils. There is no common standard, so the wedge angle and the cylinder need their own calculation.
Size any decoiler cylinder the same way
Expansion cylinders, coil car lifting cylinders and hydraulic press arms all follow F = p ร A ร ฮฒ, where ฮฒ is a load factor for losses: 0.8 is a common value and 0.6 suits demanding cases. When a cylinder retracts, only the ring area around the rod works.
To deliver 10 t (98 kN) at 140 kgf/cmยฒ (13.7 MPa) with ฮฒ = 0.8, the minimum bore is 107 mm. The next standard 125 mm bore gives about 135 kN when pushing and 104 kN when pulling with a 60 mm rod.
Two design lessons come from this kind of check. Re-calculating an earlier wedge mandrel showed that its cylinder was larger, and its structure more complex, than the load required. And a thicker mandrel shaft is not automatically better: its diameter should come from a finite-element check of the actual load case, because an oversized shaft adds cost to the bearings, the frame and everything around it. For coil ID range and support checks, see the hydraulic decoiler mandrel expansion guide and the hydraulic decoiler machine range.
05Press arm
Press arm: three questions before you add one
Each of these questions can be answered from the material data before the press arm is designed.
Not always. On many lines the roller stays lifted and the loop's own weight holds the outer wraps: a 2 m loop of 1.0 ร 1,200 mm steel weighs about 18.8 kg (185 N). If that is more than the force needed to keep the wraps from springing open, continuous hold-down is unnecessary. Thick, stiff or springy material usually needs it.
Soft strip can be threaded by hand, but that puts the operator's hands near a rotating coil and a moving strip end, and we do not recommend it. For stiff or heavy strip, a powered threading arm or peeler lifts the strip head and guides it into the straightener.
For the same bore, hydraulic pressure delivers 20 to 30 times the force of shop air. More force is not automatically better: excessive pressure and the extra hydraulic equipment bring their own drawbacks. The deciding value is the roller force the material needs.
In the example, a roller force of 155 N at 910 mm from the pivot, with the cylinder joint 386 mm from the pivot, needs 365 N at the cylinder. At 0.49 MPa (5 kgf/cmยฒ) that is a bore of at least 31 mm before any load factor, rounded up to a standard size; the cylinder calculator above does the same with the shop-air preset.
If the press roller is powered, match its surface speed to the payoff speed. In the example, a 1,400 rpm motor through a 1:20 reducer and 14 / 25 sprockets on a 200 mm roller gives 24.6 m/min against a 24 m/min requirement. For press arm functions, selection and safety limits, see the decoiler press arm guide; press arm configurations are also shown on the motorized and non-powered decoiler pages.
06Side guides
Side guides: size the motor from a person's hand force
Side guides, also called coil keepers, stop the coil from moving along the mandrel. Early designs were set by handwheel; later ones use a motor, and some adjust themselves to the coil width automatically.
When a manual design is converted to a motor, the handwheel force is a practical reference: ergonomic guidance puts a suitable handwheel operating force at 40โ150 N. Taking the upper limit gives a conservative motor.
| Worked example | Value |
|---|---|
| Lead screw | ร30 mm, 6 mm lead, 1,200 mm long, 100 mm nut |
| Drive: 1,400 rpm motor, 1:10 reducer, 30 / 15 sprockets | 280 rpm at the screw |
| Travel speed and full stroke | 1.68 m/min ยท 1,100 mm in 39 s |
| Torque from 150 N on a 200 mm handwheel | 15 Nยทm |
| Calculated / selected power | 0.44 kW / 0.75 kW |
Because 150 N is already the top of the range, the result is conservative, and a 0.4 kW motor would also do the job. For a heavy-duty keeper design, see the T-slot coil retainer arm.
07Cheat sheet
Every module on one page
| Module | What really decides it | Key relation | Worked example |
|---|---|---|---|
| Coil car drive | Rail installation and wheelโrail resistance, then weight | P = Wยทgยท(ฮผ+k)ยทv รท ฮท | 2.06 kW โ 3 kW motor |
| Mandrel drive | Payoff mode and control, then line speed, then weight | P = Jยทฯยฒ รท (t_aยทฮท); check speed at the ID | 12.8 kW fixed-speed vs 1.3 kW with a VFD |
| Wedge expansion | Wedge angle above the friction angle | ฯ = arctan ฮผ, ฮฑ > ฯ | 14ยฐ wedge, 8.5ยฐ friction angle |
| Any cylinder | Force, pressure and load factor | D = โ(4F รท (ฯยทpยทฮฒ)) | 10 t at 13.7 MPa โ 107 mm โ 125 mm bore |
| Press arm | Loop weight versus hold-down force; roller speed | F_cyl = F_rollerยทL_r รท L_c | 155 N โ 365 N โ at least 31 mm bore |
| Side guides | Hand force of 40โ150 N | P = Fยทrยทฯ | 0.44 kW โ 0.75 kW motor |
08Sizing data
What to send for a decoiler sizing check
- Coil: material grade and yield strength, thickness and width range, ID and OD, maximum coil weight.
- Line: required speed, downstream machine (straightener, feeder, press or shear), payoff mode, and whether the payoff is switched by a loop sensor or speed-controlled by a VFD.
- Loading and site: coil car or crane, rail length and foundation condition, space around the decoiler.
- Options: press arm, threading support, side guides, outboard support and brake.
The decoiler selection worksheet lists the same data as a form, and the 3-in-1 decoiler straightener feeder shows how the decoiler section is combined with straightening and feeding.
Scope of these calculations: they are first-pass design checks that show how each module responds to coil, line and site data. They do not include every load, safety factor or duty cycle. Final motor, cylinder and structural sizes are confirmed for the specific machine and line.
HENLI decoilers
Find the decoiler for your coil
Each HENLI decoiler family covers its own range of coil weight, width and payoff mode. Open a product page for its configurations and quotation checklist.
?FAQ
Decoiler design questions
How is decoiler motor power calculated?
For the mandrel drive, start with the power needed to accelerate the full coil: P = J ร ฯยฒ รท (t_a ร ฮท), where J is the coil's moment of inertia and ฯ the mandrel speed. Check the strip speed at the coil ID, then add friction, the strip pull into the next machine and the duty cycle. The control method matters: a fixed-speed motor switched by a loop sensor needs far more power than a VFD that follows the line speed.
Why does line speed matter more than coil weight?
Acceleration power grows with the square of the rotating speed but only in proportion to the coil mass. Doubling the speed roughly quadruples the acceleration power; doubling the coil weight roughly doubles it.
Why can the same coil car work at one plant and stall at another?
The drive force depends mainly on rail installation and wheelโrail resistance, not on the coil. In the worked example, the rail installation allowance is twice the rolling-resistance term, so a less level or less straight track can decide whether the motor is large enough.
What wedge angle should a wedge-type mandrel use?
An angle above the friction angle, which is arctan ฮผ: 8.5ยฐ for a friction coefficient of 0.15. The worked example uses 14ยฐ. Smaller angles give more radial force for the same cylinder but less margin against the segments sticking.
Does every decoiler need a press arm?
No. If the weight of the strip loop keeps the outer wraps in place, the press roller can stay lifted while the line runs. Thick, stiff or springy coils usually need continuous hold-down, and powered threading support is safer than threading stiff strip by hand.
Need a Decoiler Sized for Your Coil and Line?
Send the coil weight, width, thickness, ID / OD, material, line speed, payoff mode and loading method. HENLI will check the coil car, mandrel drive, expansion and press arm against your data and recommend a configuration.
