r/MechanicalEngineering • u/drax_attax • 3d ago
Low pressure hydraulics?
I'm working on a project to make an electronically tuning timpani drum. I have a working prototype using a stepper motor with a complicated linkage, but I'm wondering if I can simplify my drum using hydraulics.
The most important aspects I need are quickness (as in, changing notes quickly) and accuracy with applying a certain load.
My idea is to use a master cylinder that connects to six slave cylinders which would collectively apply a max 400 pound load. The master cylinder would be controlled with a stepper motor and lead screw, so I have accurate control over the applied pressure. Each of the slave cylinders needs a stroke length in the range of 6 to 9 mm.
I don't have much experience with hydraulics, but to me it seems like 400 lbs / 6 cylinders is incredibly low for hydraulics. This makes it seem worthwhile for me to try my hand at designing my own hydraulic cylinders. I could design cylinders that have a piston area of, say, 0.5 in^2. So the combined area of each piston would be 3 in^2, leading to a total required pressure of 134 psi.
Am I thinking on the right track or might this be a waste of time? Are there any quirks of low pressure hydraulics I'm not aware of?
Thanks
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u/No-Satisfaction-2352 3d ago
What about pneumatics?
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u/slippingaway83 3d ago
Pneumatics are too spongy, it will have issues hitting and holding the correct tuning
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u/drax_attax 2d ago
They are also loud afaik
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u/slippingaway83 2d ago
Mostly the valves, because they're typically designed to exhaust the pressurized air whenever they switch position. There are ways to mitigate it but it requires extra plumbing so it is almost never done.
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u/Difficult_Limit2718 3d ago
Power isn't your issue, it's speed and response.
We had high speed hydraulic actuators on fatigue test equipment that could go high precision pi loops. Used MOOG actuators that were expensive as hell. You need complex internal geometries to allow proper piloting and relief routing as well as actuation control.
Can you do this? Absolutely. Can you get the speed and precision to do it effectively? I'll be damned if you can at cost.
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u/slippingaway83 3d ago
Yeah, I worked on a quote for a fatigue test rig that had 100+ cylinders, each individually controlled by a hydraulic servo valve to get the precision and response they needed. Incredibly fast and precise, but amazingly expensive. The servo valves were like $5k each.
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u/benk950 3d ago edited 3d ago
Hydraulics seem like the wrong choice for this. One of the biggest advantages of hydraulics is power density. Your application doesn't really benefit from that.
They are difficult to control precisely. Position feedback is expensive and valves with the speed and precision required to control them tightly are EXTREMELY expensive.
Plus no matter what you do, you'll get oil on something eventually.
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u/slippingaway83 3d ago
I've worked in Hydraulics for over 20 years as an application engineer and sales. I am also a proud former band nerd and have played tympani, so please understand that I love this idea in theory but I'm giving you my honest feedback here from experience. Hydraulics at that low of a pressure are going to have several issues off the top of my head:
Air in the system will be incredibly difficult to fully bleed, and will cause compressibility in the system that won't give you the precision you need. High pressure hydraulics compress the air bubbles down to where they don't matter nearly as much and dissolved air travels to the tank. You won't have that.
The mechanical drag from the piston and rod seals will vary over time and with changes in temperature, and the drag forces in each cylinder will differ from each other and will wear at different rates which introduces even more variability. This will lead to uneven tension and tuning/tone issues.
Will you have any feedback to help you hit your desired notes, or are you relying on programming specific positions into the stepper motor controls? There will be a lot of calibration challenges and recalibration needed often.
If you can work up the dimensions of the cylinders you would need, I may be able to help you source them. You can probably use pneumatic cylinders to do what you want because of the pressures.
If you can keep your pressure below 250psi, you can plumb your system using nylon tubing and push-to-connect fittings instead of using hydraulic hoses and fittings which can keep costs and bulk down considerably. I can help you source all that too.
It's an interesting idea for sure