Homemade Electromagnetic Sledge Hammer: Build & Power Needs

[Albert E]

Hi,
I would like to make a homemade electromagnetic device. It would be a 5cm diameter metal cylinder inside of a metal pipe. I would use electromagnetism to lift the (heavy) steel cylinder up the pipe, and then release it. It's meant to be used as a sledge hammer.
Can I just wind a wire around the pipe? How much power do you think this will need? The cylinder weighs about 10kg and is 2ft long.

 

[Jeff Rosenbury]

This is a difficult problem.

Can you put circuitry inside the pipe? That would help.

The problem is that metal pipes don't play well with magnetism. You will get eddy currents which generate a counter magnetic field working against anything you are trying to do. You could overpower these, but with heavy losses and some heating of the pipe. There might be solutions to this problem if your application is flexible. (A plastic/ceramic pipe?, electronics on the inside? Special alloys?)

You can't just wind wires around the pipe, though something similar might work if your hammer is iron and the pipe is diamagnetic (nonmagnetic). (An iron pipe would be much harder to do.)

How much power you need depends on how far you want to move your hammer. Energy = Force x Distance. Plus you will need extra for losses. Figure double for moderately efficient applications, much more if you go the brute force method.

 

[Albert E]

I could use a PVC pipe instead if that would work better.

 

[Jeff Rosenbury]

I could use a PVC pipe instead if that would work better.

Yes, it would.

Winding wire will likely work with a soft iron hammer. This will make a solenoid.

There may be better designs, but for quick and simple, this might be the answer.

You will also need a good DC source and some moderately heavy wire for the winding. Be careful not to overheat the coil. That could be a fire hazard.

There are some equations you'll want to run that depend on your application. Figuring the effects of "air" gap caused by the thick PVC is beyond my skill, so I might try some experimentation were I you. (Oh good, experiments. )

 

[stedwards]

Normally a lifting magnet is made with the geometry show in the picture below.

The center is one magnetic pole, the outer, silvery ring is the opposite pole, and in between are the windings.
You want a flat surface on the top of the hammer to most efficiently close the magnetic circuit, and gain the greatest lifting force for a given magnet size.

Some lifting magnets are permanent/electromagnets. This means that there is a permanent magnet to do the lifting. To drop the hammer, a pulse of current is delivered through the coil that cancels the field generated by the electromagnet to release the hammer.

Is the hammer made of steel? Do you have access to machinery to fabricate something similar to the above picture?

 

[Jeff Rosenbury]

Normally a lifting magnet is made with the geometry show in the picture below.

The center is one magnetic pole, the outer, silvery ring is the opposite pole, and in between are the windings.
You want a flat surface on the top of the hammer to most efficiently close the magnetic circuit, and gain the greatest lifting force for a given magnet size.

Some lifting magnets are permanent/electromagnets. This means that there is a permanent magnet to do the lifting. To drop the hammer, a pulse of current is delivered through the coil that cancels the field generated by the electromagnet to release the hammer.

Is the hammer made of steel? Do you have access to machinery to fabricate something similar to the above picture?

My understanding is that he wants a solenoid to move the hammer without opening the pipe. Otherwise, why not just use a hook?

But perhaps I'm wrong.

 

[stedwards]

My understanding is that he wants a solenoid to move the hammer without opening the pipe. Otherwise, why not just use a hook?

But perhaps I'm wrong.

yeah, you're probably right. If so, that would be a lot of copper.

 

[Nidum]

Rather than use a one winding solenoid you could use a poor man's linear motor . Several windings packet switched would pull the hammer up in a controlled manner . Need position feed back but that's easy enough .

Problem though is that the whole device is going to be quite heavy - is this a fixed or hand held application ??

 

[Nidum]

A common way of doing this sort of thing is to use a poor man's linear motor - several solenoid windings along length of outer pipe packet switched to draw armature up in a controlled manner . Ideally needs position feed back but can usually be made to work in practice without feedback after a bit of experimental setting up .

 

[Baluncore]

It's meant to be used as a sledge hammer.

The problem is that the cylindrical pick, 50mm diameter, 2 foot long, approx 10kg, must drop from quite some height to hit like a sledge hammer. An efficiently used 18lb sledge usually falls from about 6 feet above the target. Power requirement will depend on the height of the drop multiplied by the hammer hit rate. Take a look at the hydraulic hammers used on excavators to break rocks and concrete.

As per Nidum's suggestion, you may consider driving the pick with a linear motor where alternatively reversed permanent magnets make alternating poles. The pick should be connected to the motor through a resilient coupling since if directly coupled, the impact will shatter the magnets. Then wind three phases of coil on the ceramic or PVC pipe, repeating each NS+SN+NS+SN pitch of the magnets. The pick can then be both lifted and thrown down. A hall sensor then finds position of motor and so can drive the motor with optimum phase.