# Pressing and strapping of a step-wise object

I am not a mechanical engineer and I would appreciate a bit of help on the following matter.

Namely, suppose there is a step-wise object with shape and dimensions provided in Fig. (a). In fact, the object is composed of a multitude of thin metal sheets placed as in the depicted arrangement. The object is to be pressed with hydraulic bottle jacks (c.f. Fig (b); there is a flat beam serving as a support and placed above the jacks) such that the pressure in the middle of the object exceeds 0.15MPa. Therefore, the first question would be:

1. How many pressing points are sufficient and what should be the force at each one of them? Assume that hydraulic jack can obtain any desired pressure/force.

After the object is successfully pressed, it is supposed to be strapped so that hydraulic jacks can be removed, while the pressure in in the object is retained. The second question arises from here:

1. If polyester straps withstanding X tons are used, how many straps are to be used? What should be the clamping (tightening) force for each of the straps?

Several remarks:

• If any approximation of the object (make it rectangular, circular, etc.) can be used, please do so.
• If there is any information or detail required for the calculation (e.g. strap width, maximum tension per mm, etc.), either make an assumption or let me know what's required.
• In case there is any literature where this subject can be studied more in-detail, please feel free to recommend it.

Many thanks

• Welecome to E.SE. If that illustration is not your own work then site policy requires that you give credit and reference to its creator. This looks like a homework exercise. Is it? Commented Dec 29, 2023 at 12:29
• No, this illustration is my own work. I will take your comment as a compliment :) Commented Dec 29, 2023 at 13:30
• Excellent work for a "not a mechanical engineer". The moderators are insisting on credit these days. Commented Dec 29, 2023 at 13:40
• Use band clamps. Wrap with rubber, then a thin sheet of HDPE, then a band clamp. That would have a consistency of +- 25 %. Straps would be iffy at =+- 50%. shop.advanceautoparts.com/p/… Commented Dec 30, 2023 at 1:10
• Unfortunately, my access to this link is denied :( As for the band clamps - these objects can be more than 0.5m wide. Is there such a big metal band clamp? Are there any polyester strap alternatives with the screw mechanism? Commented Dec 30, 2023 at 5:58

That's an interesting question - and slightly odd. Thin metal sheets should be incompressible. I assume that in this application there may be some warpage / shaping which requires compression. Anyway ...

I'm no expert on this subject but if we look at the total reactive force for 0.15 MPa (150 kN/m2) and multiply by the area we should be able to calculate the holding force required.

$$F = L \times W \times 150\ \text {kN}$$

The straps will have tension on both sides of the object so the hold-down force per strap should be,

$$F_{straps} = \frac {LW} 2 150\ \text {kN}$$

and distributing that over $$n$$ straps should give us,

$$F_{strap} = \frac {LW} {2n} 150\ \text {kN}$$

Sanity check:

L = 2 m, W = 0.4 m, n = 4

$$F_{strap} = \frac {2 \times 0.4} {2\times 4} 150\ \text {kN} = 15 \text {kN}$$

This equates to a tension of 1500 kg on each strap, so 1.5 T. (To convert N to kg divide by 9.81 - or 10 if you're like me.)

• Thanks for the answer! Application - large power transformers :) I am sure you know that magnetic core of a power transformer is build up by means of thin silicon-steel sheets. In other words, it is laminated with the aim of reducing iron losses. If core sheets are to be left without sufficient compression, transformer noise levels are not satisfactory, therefore, some force needs to be applied for keeping the magnetic sheets together. I hope this clarifies the application and the purpose of my question (even though it is still mechanical-engineering related, the product is electrical) Commented Dec 29, 2023 at 17:04
• Ah, yes. I've wound my own 100 W transformers and worked with some 1 MVA and 2 MVA oil-cooled and air-cooled versions. I would have thought that your problem had been solved many times by many companies over many decades? Commented Dec 29, 2023 at 17:12
• Certainly it had, however, these are proprietary technological protocols which are normally not shared with the public :( Commented Dec 29, 2023 at 17:13