Hydraulic accumulator and acceleration of a mass 1

[Jaguarnotredbull]

Dear Sir, Madam,


I have a problem that is proving difficult to solve. Having researched high and low I have concluded that I need some help please!!

Firstly, I would like to thank you for reading my enquiry and any help is much appreciated.

The problem I have is to do with hydraulics. The aim is to design a linear catapult that accelerates a 90kg mass with a speed of 36m/s within 7.5m of launch, and then the mass is detached.

Having conducted some initial research and designs, I have found that the use of hydraulics would be best. My idea is to use hydraulic oil that is pumped from a reservoir into storage cylinders filled with nitrogen i.e. an accumulator. The nitrogen compresses and acts like a spring, and the hydraulic oil acts like potential energy. When the required pressure has built up the rig is ready to launch. Pushing a start button opens high-speed valves at the storage cylinders and sends the oil flowing to hydraulic motors. Gears on each end of the motors turn a large planetary gearbox. Planetary gearboxes sit on either side of a cable-winding drum that turns at a certain rpm. The drum works like a big fishing reel, taking up cable as it turns. The launch cable attaches to a locking device called a catchcar that, in turn, attaches to the rig. The catchcar runs in the track and propels the train forward. Reaching the target speed 36m/s and disengages the cable from the rig after 7.5m to them close the hydraulic valves, and restart the process.

My concept should work from principle but the only problem I have is the calculations, i.e. what calculations or equations do I need to work out if my concept works to produce the desired velocity. I understand that I have
to take viscosity, pressure differences etc into consideration, but when and where?

As this is the first hydraulic unit I am designing I would appreciate if you could lead me in the right direction and if possible prove that my concept may work using your expert knowledge.

If you can think of a better way such as just using a hydraulic piston arrangement please let me know, but in
either case I hope you can help.

I appreciate your time and I would very grateful if you
could assist me.


Thanks again for your help.

Regards

jaguarnotredbull

 

[hydromech]

jaguarnotredbull...

Not an easy one...36 mtrs/sec is fast.

The basics are...

A motor not a cylinder would be required to do the job. The linear speed is prohibitive, even with piston rings in the cylinder instead of seals, the friction coefficient is probably high enough to weld the piston head to the bore of the cylinder. In any case the volumetric displacement of the cylinder would require huge ports that would make the size prohibitive.

To strike a happy medium between the speed and the torque required, I would suggest a drum diam' in the region of 2 mtrs.

This would allow a motor speed of 360 rpm, with a motor torque output of 132.3 DaNM (1323 newton/metres). This torque is based on a load of 135kg. I added 50% to the load to be sure that it could be accelerated to 36 m/s with space left to decelarate before the rig launches itself behind the projectile. It also allows for frictional losses.

The tricky bit is sizing the accumulators and the flow control valves to maintain the required pressure.

I would suggest a proportional flow control valve to ramp the flow up, maintain the flow at the required level and then ramp the flow down again to slow the rig.

The accumulator precharge needs to set so as to maintain the minimum presssure required to drive the motor.

It is tricky...but it would work...got plenty of money?

These are the basic fundamentals...you will have to fill the gaps. You can vary the speed or the torque...along with the flow and pressure...but the input power is always going to be the same. How much power you require depends on the frequency of operation.

Let us know what you think.

Regards

Hydromech.

 

[fIEROWISEGUY]

Hydromech is right in if this were something you were doing on a production design and to be used repeatedly. I suspect however, this might be something like a "Pumpkin Chuckin" machine?

Good old "F=M*A" is where you start. Figure out the required force to accelerate your mass. And not just the payload, but the trolley (or whatever it is), cable mass, gear mass, etc.

It sounds like you need some massive torque if you go the motor route. Why not use a long cylinder,and connect it to the cable with block and tackles, to multiply the velocity and distance? The Navy does this on landing arresting gear, where the cables are connected to big hydraulic cylinders. You are doing it in reverse.

And if I'm right about this being a one-time deal, Why use an accumulator and oil? Just open gas valve directly into the motor (or my cylinder suggestion if you so choose). A couple bottles of nitrogen are a whole lot cheaper than a huge accumulator and massive valves.

"fIEROWISEGUY"

 

[Jaguarnotredbull]

Hi Guys, thanks for the help.


What I have done so far is as follows.


I have re-designed my configuration and have used a piston and accumulator system similar to what was advised by fIEROWISEGUY. I have designed a system using pullys that doubles my speed and distance traveled by using half the length of the initial cylinder.


Using a stroke of 3.0m, the total force needed to overcome friction and other losses came out to be 21578N using a pressure of 10 bar, the required diameter of the piston is 24 cm and rod diameter of 8cm. I calculated this by taking acceleration of the piston to be 216m/s^2 as it is now traveling through 3 metres to achieve the desired 36m/s^2 on the track. The volume required for the cyclinder came out to be 60 litres with a flow rate of 4300 litres per minute.

I then discovered that if I use an accumulator of 207 bar and 90 litre (parker website) capacity it can achieve the desired result in the piston. But I am assuming that as the pressure is so large in the accumulator that the pressure difference will be negliable.


I am not sure whether this is correct after some of the points Hydromech pointed out.

The other alternative is to use a bungee rope but I rather use hydraulics since I will learn more by doing it.

Hydromech, your solution sounds like it could work but as the system will need to be used frequently say every 3 minutes, will the system still work. Plus, using a motor, will the desired speed be reached at the end of the track or the beginning – how do work this out? And if you can help me find out the missing gaps, that would be great. Regarding money, the design should be as cheap as possible……I hope.


I am now really confused on the best efficient next step to follow. Any chance on a full system that will be the best to use and that I can prove using equation etc?

fIEROWISEGUY, the catapult used by the navy seems also good, any idea on the details of operation and possible calculations, cheers.


Many thanks and I hope to hear from you soon.


Regards


Jaguarnotredbull

 

[hydromech]

A cylinder would be the most suitable method of accelerating the load...however cylinders are simply not designed to move at 36 mtrs/sec. The linear speed of the seals against the bore of the cylinder will cause the seal to burn away very quickly.

For safe,reliable, controlled and repeatable operation the only option is, in my opion, a hydraulic motor driven by a piston accumulator and controlled by a proportional flow control valve.

The frequency of operation dictates the the rest of the package...3 minutes does seem workable.

Jaguarnotredbull...I can only tell you how I would do it. It's your choice and your money.

Regards

 

[Jaguarnotredbull]

Hi mate,

thanks for the reply.

I see where you are coming from now. I will go forward with your method, any chance on some help with regard to the required pressures needed etc to reach the speed of 36m/s, cheers. (By the way, will the piston seals burn on 18m/s), cheers


Regards

Jaguarnotredbull

 

[MadMango]

Slight clairification: The Navy uses a hydraulic system for the arrester cables for landings, but they use a steam system for the catapults during take-off. Fast-ramping acceleration, quick recharge time.

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[fIEROWISEGUY]

That's what I said; landing arresting gear; not catapult.

Multiplying the velocity of the mass using pulleys and the cable is the same thing in reverse of this. You would have a shorter cylinder stroke and slower cylinder speed, while multiplying the velocity of your load.

Another thought about the concerns of seal velocity; use cast iron rings. Just look at the velocity and the conditions that piston rings operate at in a diesel or gasoline engine. They far exceed what we are talking about here.

If you want to talk to someone about using an elastomer type seal, I recommend you contact Tom Duke, chief engineer at Macrotech Polyseal in Utah. 1-800-453-8742. Mr. Duke is a real inovator and problem solver. Many times he has helped us develop seals and solutions to our satisfaction.

Jaguar, if you want to talk with him about this, I checked and Tom said he would welcome your call.

for what it's worth,

fIEROWISEGUY

 

[Jaguarnotredbull]

Thanks guys for your help.

fIEROWISEGUY, thanks for the info and contact name and number. As I am from Scotland is there any chance of an e-mail contact, cheers.

I will have a go on a new design, but if anyone comes up with another solution I would welcome it anytime.

Cheers guys and I hope to hear from you soon.

Regards

Jaguarnotredbull

 

[fIEROWISEGUY]

Scotland? That explains the accent.

Tom Duke's Email is TOM@POLYSEAL.COM

fIEROWISEGUY

 

[EdDanzer]

We have designed and built an acoustical signal generator that moves at over 300 inches per second using 80 psi air pressure. The trick to high speeds in fluid power is reducing pressure drop between the stored energy source and the piston driving the load. We use a 1 ½” poppet valve to dump a 4” cylinder. The seals are glass filled Teflon.
Your problem will be storing enough volume to limit pressure drop off to provide adequate force and flowing adequate volume to achieve the speed needed.

 

[Jaguarnotredbull]

HI EdDanzer,

Thanks for the message. Is there any chance of helping me solve me problem of moving 36m/s in 7.5m since it seems you have some previous experience.

Any help welcome, cheers.

Regards

Jaguarnotredull

 

[EdDanzer]

I have very little time, but can assist some. First calculate the force required to accelerate the mass (90kg) plus the mass of the parts holding and driving the mass) to speed in the specified distance. This will determine the cylinder bore. If you use a cable to multiply the force, that mass must be added to the weight to be accelerated. With the bore and stroke know the volume of fluid to extend the cylinder. This volume must be stored in containers of adequate to reduce pressure drop at the end of the stroke. The time required to accelerated to speed in minutes or fractions of a minute times the extend volume gives the flow required for valves and the storage containers.

 

[fIEROWISEGUY]

Don't forget to include the mass of the moving oil, too.

fIEROWISEGUY