My Senior design project - a "Swamp Thing" tracked vehicle clone

[Garauld]

Personally I think Garauld is reinventing the wheel...

-> what wheel is that? This has been a fun project so far and is getting interesting. The original "Swamp Thing" used, I believe, a hydrostatic system like balsam is using. Watch him drive it in the video I posted. I also think it's a small diesel powering it.

 

[bcp]

Reinventing the wheel is OK. It works. But some people seem to reinvent the flat tire.



Bruce

 

[balsam]

Wow! I’m impressed with the response. Thanks guys.
I think you would have to understand a little bit of hydraulic circuitry to understand what I am proposing.
It’s correct that I would need a tandem pump set up to achieve a zero turn radius, which is not important to me. Being able to reverse the unit and have a turn radius within two widths of the machine is fine for my needs....the same that is achieved with differential and brake steering.
I can select the displacements of the pump and wheel motors to give me any speed or torque I require, then match an engine to it. In my calculations, I have designed the system with enough torque to pull the estimated weight of the machine up a full 90% incline and then half again to pull a small fishing shack at up to 30mph. Perhaps overkill but I have a habit of designing that way.
That tandem pump from Surplus Center would have way to small a displacement for my needs and using one pump with the wheel motors in parallel is so,much simple to plumb. I could use a closed loop circuit to reverse the flow at the pump or open loop(no charge pump needed) with a simple detent positioned directional control valve to select forward and reverse.
You’re right that it might be a little noisier than a mechanical drive but the engine would sound the same and good sound baffling would be important around the pump.
I live on one of the Great Lakes and want to make this thing for ice fishing. I’m designing it to be buoyant in the chance it goes through the ice and with an escape hatch on the roof. I will use a water cooled engine so as to have a heated cab.....can’t take those -30° trips on a snow machine any more.
I could sketch the hydraulic circuit I have in mind but don’t know how to post it.
I guess I am asking if I would be achieving the same performance characteristics with my hydraulic system as I would get with an open differential and brake system.
Really enjoying your comments. Thanks!

 

[balsam]

Garold: the reason I responded to your posts is because I am very impressed with your studied approach to your project and the workmanship I see in your pictures.

 

[Garauld]

Thanks balsam. What HP and engine are you thinking of? The Echo engine is rated at 108 hp which is a LOT for this thing but it was cheap. I may drop in a NASCAR-type restrictor plate if it seems too strong.

 

[John_Mc]

If you are using hydraulics for breaks, do you have some sort of mechanical backup? I'm wondering what happens for braking if the engine dies, or if you want to shut it off while sitting on a hill.

 

[Garauld]

If you are using hydraulics for breaks, do you have some sort of mechanical backup?

->At this time, I do not have a scheme in place for a parking brake. I could easily rig one up by using a spring-loaded arm that could be dropped into the unused CV axle tripod opening. I welded up the diff gears so that arm would securely lock the drivetrain. I thought with a diff feeding another diff, the reduction along with putting the auto trans in park would be sufficient to hold it on most any hill. Do you always use a parking brake?

 

[balsam]

First, the hydraulics gives a certain amount of inherent braking as the inertial forces causing the motors to act like a pump has to drive the pump as a motor. Also a simple two way directional control closing off the fluid leaving the two motors positively prevents any motion.
Garauld, using the premise of 550 ft.lbs./second and an efficiency of 85% I calculated the horsepower I require to about 36hp. Surplus Center has a beauty of a brand new diesel engine they have in stock but it is only 27hp. Not cheap at $2800 and would be a little under my power requirements unless I can somehow reduce the weight of my unit. I do have a line on some gasoline engines also...lots around the 25 hp. range but air cooled and too low in hp. A small three cyl. liquid cooled engine would be perfect.
There is no fear of overpowering a hydraulic system as they are all protected by a relief valve and actuator forces can be limited that way.
You could probably use an adjustable spring loaded “slip clutch” to achieve the same thing. I have one on my tractors PTO shaft in case I pick up a rock with my snow blower.
It would be nice to add a second fixed displacement pump for auxulary hydraulics for neat stuff like a ice fishing hole auger or even a snow plow.
I have a lot of hydraulic motors and other stuff. I am used to doing plumbing with metal tubing rather than hoses and have tons of that stuff left over from other projects.
I am finishing a kitchen right now but hope to get at this soon as my winter project.

 

[strantor]

Great! I have my design pretty well worked out except for the steering. I have a background in hydraulics, so I have decided to go that route.
Most guys seem to use an open differential with brakes and from I read it痴 a simple reliable system. I am pretty sure this was the system used on the Imp, way back.
I think I have worked out a very simple hydraulic circuit by using a variable displacement pump and runnning the two track motors via of a parallel circuit. Instead of using brakes I would, with a valve, restrict flow on side or the other to slow(or stop) one track or the other, causing the turn. The flow not going to the restricted side would go to other, speeding it up as in a mechanical differential. I couldn稚 achieve 奴ero turn radius with only one pump but that痴 not impotyant to me.
In my mind, and please correct me if I知 wrong, this would work exactly like a differential system without any brake pads to wear out or freeze up.
I remember having to pull very hard on the joy sticks to engage the brakes on the Imp, which would not be the case with a hydraulic valve.
Achieving reverse would be an inherent part of the pump itself, or a directional control valve could be used also.
I know introducing hydraulics creates a few problems in terms of efficiency and cold oil etc. but think the syplicity, reliability and compactness is a fair trade off.
I would appreciate any thoughts you might have. Too bad we couldn稚 chat about this over a beer.

Hydraulic braking can be tricky. When you have a load with lots of inertia (like a vehicle) on a hydraulic motor and you cut flow, that motor turns into a pump. If that pump (drive motor) is sucking against a closed valve, it creates cavitation. Little vacuum bubbles that implode inside your drive motor and cause damage. If that pump (drive motor) is trying to pump fluid through a closed valve, seals blow, motor housings can explode, it's never a happy ending. For this reason a special motor control hydraulic valve is typically used, which has a couple of pressure reliefs in it, allowing the fluid trapped between the valve and the motor to flow in a loop, as it's being pumped by the motor. This spool, and the way it works, limits the amount of hydraulic braking that you can achieve. I believe this is the reason why hydraulic drive is not used for high speed vehicles, esp. skid steers.

 

[balsam]

All true. The fluid has to be metered as it leaves the motor and the high pressure caused by the motor working as a pump is usually handled by “across the line reliefs”. I designed a number of ski lifts driven by hydrostatic transmissions and they need to stop in a hurry in case of an emergency. They are braked this way.
I don’t know if braking is why hydraulics is not used on higher speed vehicles. I always understood the reason to be the poor efficiency you get by moving a viscous fluid quickly through a system. The thicker the fluid, the poorer the efficiency. On the other hand less viscous fluids allow more internal leakage, pump slippage, etc, and the lost horsepower is tuned into heat, creating another issue.
You’re right that it is usually used on slower vehicles to gererate high torque or force.
I appreciate your thoughts.
I think i will start a link in the hydraulic section to see if I can get some feedback on the “specific” circuit I plan on using. Before I do, though, I better figure out how to post a schematic of it on this site....i’ll Get my 11 year old grandson to help me.