Hydraulic Dune Buggy Design: Is My Math Correct?

[larkinja]

How does series work in a hydraulics? If two motors are put in series, do the motors still put out the same amount of torque and rpm. Would it be possible to have the two rear motors in series creating a posi effect on the rear, and then have the front 2 in parallel giving a open differential effect?

Any chance you could give me an idea of what all the components should cost, the way you have it drawn?

 

[larkinja]

Actually torque would be half or rpm doubled wouldn't it?

 

[larkinja]

Also, you could look at a series/parallel circuit. Run the right rear to the left front and vica-versa. Then the motors themselves act as a flow divider. you would need both corner wheels to lose traction in order to have a runaway condition. It's not a perfect setup flow wise because there is some loss of fluid to the case drain for each motor and the rear wheels will tend to push the front but it would work with minimal hydraulics. You will still need some check valves in between to protect the front motors from cavitation.

How would this be plumbed?

 

[drankin]

How would this be plumbed?

There are trade-offs when you try to cut corners. Your power to the drive motors would be half and the max speed would double in this configuration with a given size pump.

One side of the pump goes into one motor, from that motor to another motor, out to the other side of the pump.

 

[drankin]

To compensate, you would double the motor size.

 

[larkinja]

would it be possible to put all 4 motors in parallel with a flow control at each motor. Could this limit the flow to one individual motor, forcing the remaining flow to the rest of the motors?

Just a thought, trying to come up with some alternatives.

 

[drankin]

would it be possible to put all 4 motors in parallel with a flow control at each motor. Could this limit the flow to one individual motor, forcing the remaining flow to the rest of the motors?

Just a thought, trying to come up with some alternatives.

You'll never get them all set exactly. It will make the system very inefficient and possibly give you a heat issue,.

 

[larkinja]

You'll never get them all set exactly. It will make the system very inefficient and possibly give you a heat issue,.

Do you see any possible alternatives, or should we just continue the way we had planned? If so, do you have some examples of the flow dividers, and the valves used in the circuit so I can start pricing them out?

What else has to be put into the circuit yet?

One other question. Is the pump going to control the direction of the motors, or are you talking about using controls outside the pump to control the direction. Do rotary flow dividers work in 2 directions?

 

[larkinja]

Here is a picture of the frame the way it sits. Haven't done much to it till we figure out what components we are using. Once we figure out the hyd motors, we can build the suspension.

 

[drankin]

Do you see any possible alternatives, or should we just continue the way we had planned? If so, do you have some examples of the flow dividers, and the valves used in the circuit so I can start pricing them out?

What else has to be put into the circuit yet?

One other question. Is the pump going to control the direction of the motors, or are you talking about using controls outside the pump to control the direction. Do rotary flow dividers work in 2 directions?

Google "hydraulic gear flow divider". Casappa has a good selection in our flow range.

Typically there is a hot-oil "flushing" circuit that circulates oil thru the pump case. I haven't added that yet. Usually you include the motors in the flushing circuit too but I think we can go without that.

The pump controls the oil direction as the swash plate is tilted. Max tilt is max flow in one direction then as it tilts toward zero flow is reduced until it crosses 0deg then flow begins to flow the other direction while flow enters the pump from the opposite port (from the motors). The pump swash is controlled by the pilot valve. In this case the pilot valve would be a joystick or footpedal.

The gear flow dividers are bidirectional (typically).

 

[drankin]

Here is a picture of the frame the way it sits. Haven't done much to it till we figure out what components we are using. Once we figure out the hyd motors, we can build the suspension.

Nice looking frame! How much does the engine weigh?

 

[larkinja]

Nice looking frame! How much does the engine weigh?

Thanks. The engine fully dressed weighs about 500lbs. The pump 220lbs, total of 4 motors, 84lbs. Wheels and tires, 120lbs. Frame so far weighs 180lbs but that will go up. Originally we planned 2000-2500lbs with 2 adults, so we'll see how close we come.

 

[drankin]

Thanks. The engine fully dressed weighs about 500lbs. The pump 220lbs, total of 4 motors, 84lbs. Wheels and tires, 120lbs. Frame so far weighs 180lbs but that will go up. Originally we planned 2000-2500lbs with 2 adults, so we'll see how close we come.

Do you use any 3D CAD softwares?

 

[larkinja]

Do you use any 3D CAD softwares?

No, not yet. I have autocad inventor on my laptop, but haven't tried anything with it yet. I own a sign company, and do most of the designing here, so I am pretty good with design software, I just need to take some time and learn autocad. Pretty much all of the concepts, I sketch out on paper, and some parts I draw in CorelDraw. When we have parts laser cut, I use Corel, and convert to a dwx.

 

[drankin]

A manifold example.

 

[larkinja]

A manifold example.

Gotcha, now the flow dividers will be part of that, or the output of the flow dividers will go into the manifold?

 

[drankin]

Gotcha, now the flow dividers will be part of that, or the output of the flow dividers will go into the manifold?

Yeah, we could make the manifold attach to the flow divider with short sections of tube or flange adapters.

 

[larkinja]

Do you think this could help us in any way? Looked interesting. Didn't know if it could simplify our design in any way. It can handle 52gpm and 6000psi.

 

[drankin]

Do you think this could help us in any way? Looked interesting. Didn't know if it could simplify our design in any way. It can handle 52gpm and 6000psi.

That's a neat circuit but since we are running four motors in parallel we would need 3 of them in a cascaded arrangement. And you wouldn't be able to switch it on the fly. It would be like an older 4wd pickup to where you have to come to a stop, switch it, and then run. And then keep your speed low and watch your fluid temperature.

Looking at this did give me some ideas on how to restrict a runaway wheel without using a flow divider and still run all motors in parallel. We could use pressure compensated flow controls on the motor outlets that are set very high to where they don't engage unless the flow is excessive. The issue is that the dynamics will be unpredictable and that can be scary at high speeds. There could be some pressure spiking and lurching of the wheels as the system tries to stabilize causing a loss of traction and control.

A gear flow divider is the best way to go in my opinion.

 

[larkinja]

That's a neat circuit but since we are running four motors in parallel we would need 3 of them in a cascaded arrangement. And you wouldn't be able to switch it on the fly. It would be like an older 4wd pickup to where you have to come to a stop, switch it, and then run. And then keep your speed low and watch your fluid temperature.

Looking at this did give me some ideas on how to restrict a runaway wheel without using a flow divider and still run all motors in parallel. We could use pressure compensated flow controls on the motor outlets that are set very high to where they don't engage unless the flow is excessive. The issue is that the dynamics will be unpredictable and that can be scary at high speeds. There could be some pressure spiking and lurching of the wheels as the system tries to stabilize causing a loss of traction and control.

A gear flow divider is the best way to go in my opinion.

Ok, just a thought.

 

[larkinja]

Wondering if you have time for an offshoot question?

I think I am going to use an analog driver card to control the pump. I've been working with the card engineers to figure out the current ratings and such, its got the basics, adjustable ramping, separate adjustments for up and down. Adjustable dithering. Nothing fancy, but should work. It takes its input from a simple potentiometer, so this will give us huge flexibility in what we choose to use for a "throttle" device.

My question is about steering. We want to use a steering cylinder and a valve of some sort. Maybe 2 proportional valves, one for each direction, or some sort of bi-directional valve. Tonight I was looking through this device. Seems more complicated than it needs to be but it seems like the right idea

http://www.sauer-danfoss.com/stellent/groups/publications/documents/product_literature/520l0521.pdf

Do you have any experience with anything like this? I was hoping there would be some sort of basic analog device that would allow us to control the steering in a similar way as the pump control? I know I can use an orbital valve, but I don't want a traditional steering wheel. I know we can get a potentiometer joystick for under $100, so we may still play around with that idea if we can use a potentiometer for steering. Another thought if the joystick is to weird to drive is a yolk from an airplane. I took about 30 hours of private pilot lessons and the feel of the airplane controls is pretty cool. The thought would be that just a 90 degree turn to the left of the yolk would be a full wheel turn to the left, etc... Throttle could then be a thumb lever or a twist grip or a pedal on the floor with a pot box like what is used on a golf cart.

Anyway, the basic question is how can we steer with a potentiometer. And I mean fairly innexpensively, I realize there are some pretty amazing technologies that can be used, but we just don't want to spend a fortune here. We can always upgrade down the road.

The other thought is about reverse. When the pump reverses the flow to make the vehicle back up, seems the steering will be backwards, so I am guessing we would have to somehow reverse the steering direction when in reverse. Oh, for reverse, the card will have a couple relays that will reverse the polarity going into the valve. So putting the vehicle into reverse will require flipping a switch. I am thinking this switch could signal the steering to reverse as well maybe.

 

[drankin]

The main hydrostat pump will not provide oil for steering. This is never done because steering requires a dedicated oil source. You could oversize the make-up pump that is part of the hydrostat unit but even this is not a safe design for a steering circuit. Ideally, you want to piggy-back a small dedicated pump to the make-up pump.

Steering systems are a specialized dynamic application. There has been a lot of development in hydraulic steering to get it tuned and reliable.

The best and inexpensive way to steer would be with a traditional automotive power steering system. If we go custom hydrualic or electric over hydraulic we can use a joystick and performance will depend on how much you want to spend. A simple system will give you "bump" steering. You will bump the joystick in the direction you want to turn as opposed to holding the joystick in a turned position. Kind of like steering with buttons. How far you hold the joystick will only control how fast it turns not into what position it goes to. Not very natural. To have joystick angle position correlate with wheel turn angle would require an electro-proportional closed loop system. Very expensive. Or go with the Sauer valve systems but they are designed for a steering wheel and are probably more than you want to spend.

 

[larkinja]

The main hydrostat pump will not provide oil for steering. This is never done because steering requires a dedicated oil source. You could oversize the make-up pump that is part of the hydrostat unit but even this is not a safe design for a steering circuit. Ideally, you want to piggy-back a small dedicated pump to the make-up pump.

Steering systems are a specialized dynamic application. There has been a lot of development in hydraulic steering to get it tuned and reliable.

The best and inexpensive way to steer would be with a traditional automotive power steering system. If we go custom hydrualic or electric over hydraulic we can use a joystick and performance will depend on how much you want to spend. A simple system will give you "bump" steering. You will bump the joystick in the direction you want to turn as opposed to holding the joystick in a turned position. Kind of like steering with buttons. How far you hold the joystick will only control how fast it turns not into what position it goes to. Not very natural. To have joystick angle position correlate with wheel turn angle would require an electro-proportional closed loop system. Very expensive. Or go with the Sauer valve systems but they are designed for a steering wheel and are probably more than you want to spend.

Okay, you're right again. Okay, well I am familiar with using a steering valve, an automotive power steering pump and a double acting cyclinder for hydraulic steering. The thought crossed my mind that the larger the steering valves displacement is, the less number of turns it requires to move the cylinder a full stroke, right? Theoretically, if the displacement is high enough then would full steering be achieved with less that one rotation of the valve? Is there some way to calculate this? I think automotive power steering pumps generally create 2.5 or 3 gpm at around 900-1200 psi. This would already be on the engine, we haven't taken it off yet, so would be easy enough to use. I'm guessing this is probably going to be our best bet isn't it?

I like the idea of small movements on the steering wheel. Never having to take your hands off to make a full turn is the goal we're after. I was hoping a joystick would work, but it's looking less and less like that will work.

HAve any ideas? Or know a way to calculate the displacement to get the most travel from a small turn?

 

[drankin]

Okay, you're right again. Okay, well I am familiar with using a steering valve, an automotive power steering pump and a double acting cyclinder for hydraulic steering. The thought crossed my mind that the larger the steering valves displacement is, the less number of turns it requires to move the cylinder a full stroke, right? Theoretically, if the displacement is high enough then would full steering be achieved with less that one rotation of the valve? Is there some way to calculate this? I think automotive power steering pumps generally create 2.5 or 3 gpm at around 900-1200 psi. This would already be on the engine, we haven't taken it off yet, so would be easy enough to use. I'm guessing this is probably going to be our best bet isn't it?

I like the idea of small movements on the steering wheel. Never having to take your hands off to make a full turn is the goal we're after. I was hoping a joystick would work, but it's looking less and less like that will work.

HAve any ideas? Or know a way to calculate the displacement to get the most travel from a small turn?

I just don't have that much experience with steering circuits to help you there. If you could go with higher pressures and a smaller diameter steering cylinder then in theory you could turn more with less flow and still have adequate steering force.

 

[larkinja]

I just don't have that much experience with steering circuits to help you there. If you could go with higher pressures and a smaller diameter steering cylinder then in theory you could turn more with less flow and still have adequate steering force.

OK, no problem, I appreciate everything you are doing for us!

 

[ucom]

Very interesting thread. I did have a question, larkinja a few posts back suggested using a Bucher Hydrostatic Differential Lock Valve, I had an idea. Why couldn't you use it in conjunction with a set of solenoid valves to achieve a 4WD/2WD system. By opening or closing the valves with the brake pedal (much the same way your break lights come on) you could by pass the whole drive system and make it so you could use the disk brake idea. Also, in essences with the solenoid valves open it acts like a clutch in a normal car, with them closed the flow is diverted to the drives. In addition you could make the vehicle a 2WD (front wheel or rear wheel drive) or 4WD with the flick of a switch. I came up with a VERY simple layout sketch.

 

[larkinja]

Very interesting thread. I did have a question, larkinja a few posts back suggested using a Bucher Hydrostatic Differential Lock Valve, I had an idea. Why couldn't you use it in conjunction with a set of solenoid valves to achieve a 4WD/2WD system. By opening or closing the valves with the brake pedal (much the same way your break lights come on) you could by pass the whole drive system and make it so you could use the disk brake idea. Also, in essences with the solenoid valves open it acts like a clutch in a normal car, with them closed the flow is diverted to the drives. In addition you could make the vehicle a 2WD (front wheel or rear wheel drive) or 4WD with the flick of a switch. I came up with a VERY simple layout sketch.

The biggest problem so far is the flow rate. The pump is capable of at least 75gpm, and the valve can't handle that much. Actually I am having trouble even finding a gear divider that can handle that flow. The thought at one time was to gang 2 smaller dividers together to separate the front from the rear and handle the flow, then use the differencial lock valves at each axle. Although we're thinking the original plan might still be the best. That is if we can afford the large 4 port rotary divider.

 

[drankin]

The biggest problem so far is the flow rate. The pump is capable of at least 75gpm, and the valve can't handle that much. Actually I am having trouble even finding a gear divider that can handle that flow. The thought at one time was to gang 2 smaller dividers together to separate the front from the rear and handle the flow, then use the differencial lock valves at each axle. Although we're thinking the original plan might still be the best. That is if we can afford the large 4 port rotary divider.

What are you getting for pricing on the flow dividers? They shouldn't be very expensive. All a gear flow divider is 4 gear motors with a common shaft, a common port on one side and individual ports on the other.

 

[ucom]

What about this for a drive system. See Sketch. Remove the complex valve system all together and replace it with a simple high flow selector valve, and then use two motors with a mechanical link to provide the no slip differential on the two sides, this gets rid of the need for a high flow complex valving system and still gives you all the traction as before. The only draw back I see is that you would need to more drives in the system.

 

[larkinja]

What about this for a drive system. See Sketch. Remove the complex valve system all together and replace it with a simple high flow selector valve, and then use two motors with a mechanical link to provide the no slip differential on the two sides, this gets rid of the need for a high flow complex valving system and still gives you all the traction as before. The only draw back I see is that you would need to more drives in the system.

So, by drives you mean actually use 2 hydraulic gear motors with the shafts coupled together? Interesting idea. Is still think the flow rate is going to be a problem though. Even with the flow divided into 2 equal streams, that is still sending almost 40gpm into a single motor, and most motors have a 20-25gpm max. Correct me if I'm wrong, but basically that is what a rotary flow divider does isn't it?

 

[larkinja]

Drankin, I have a couple questions about the current schematic. One, the high speed circuit sounded like a great idea, but I'm wondering if it is necessary? The motors can only take 25gpm max, so diverting all the flow will just overspeed the motors anyway right? Can you think of a reason to keep it? Other than doing donuts, I can't think of any reason to have just 2 wheel drive anyway.

2nd question. I realized that in the text you have reverse function listed in the manifold. What does this mean. I thought we would be reversing the pumps swash plate and simply reversing the flow?

With this in mind, could we use the rotary flow divider as you have it, then just have a small solenoid valve to go between the two front motors, and one between the two rear motors? Seems like this would give us selectable lock on the rear, and a selectable lock on the front. With low flow going between the two motors, wouldn't this give us a limited slip effect without generating to much heat since it is just a lower flow balacing effect?


Let me know what you think.

 

[larkinja]

Another question. With the rotary flow divider, can the output from the pump have a large tee in it going to 2 2port rotary flow dividers? Seems it might be easier to find 2 port rotary flow dividers that can handle 40gpm that 1 4 port that can handle 80gpm.

Actually I am having tough time finding any the even come close in flow. Of the manufacturers that make them most seem to top out at 10 or 12 gpm. Do you have any suggestions for brands to look at?

 

[drankin]

Drankin, I have a couple questions about the current schematic. One, the high speed circuit sounded like a great idea, but I'm wondering if it is necessary? The motors can only take 25gpm max, so diverting all the flow will just overspeed the motors anyway right? Can you think of a reason to keep it? Other than doing donuts, I can't think of any reason to have just 2 wheel drive anyway.

2nd question. I realized that in the text you have reverse function listed in the manifold. What does this mean. I thought we would be reversing the pumps swash plate and simply reversing the flow?

With this in mind, could we use the rotary flow divider as you have it, then just have a small solenoid valve to go between the two front motors, and one between the two rear motors? Seems like this would give us selectable lock on the rear, and a selectable lock on the front. With low flow going between the two motors, wouldn't this give us a limited slip effect without generating to much heat since it is just a lower flow balacing effect?


Let me know what you think.

No prob, let's lose the hi-speed valves.

With the reverse function I have a series of "pilot to close check valves". The check valves are to protect the motors and flow divider from cavitation (self destruction). But in order to reverse you have to close those check valves so that the flow goes back through the motors instead of around them (and thru the flow divider going forward). I have it setup to protect the motors and flow divider in both forward and reverse. It looks big on the schematic but it will be a compact little manifold circuit.

As is shown in the schematic there is already an orifice between the flow divider motors for the front and rear sections. This provides the limited slip function. I could include this feature in the anti-cav manifold if needed. I based the schematic symbol off the "D series" Haldex rotary flow divider: http://www.haldex.com/Global/Hydraulics/Product%20Catalogs/flow_div_1205.pdf

 

[drankin]

Another question. With the rotary flow divider, can the output from the pump have a large tee in it going to 2 2port rotary flow dividers? Seems it might be easier to find 2 port rotary flow dividers that can handle 40gpm that 1 4 port that can handle 80gpm.

Actually I am having tough time finding any the even come close in flow. Of the manufacturers that make them most seem to top out at 10 or 12 gpm. Do you have any suggestions for brands to look at?

Using a tee is not a problem if that's how we have to go. We still need to add pressure filters on the two pump ports. Gear motors tend to create a lot of contamination as they wear and we will want to capture those particulates before they get to the pump.

I'll ask around for higher displacement flow dividers. If we don't find any that are reasonable we could go with a couple of two section dividers.

 

[larkinja]

No prob, let's lose the hi-speed valves.

With the reverse function I have a series of "pilot to close check valves". The check valves are to protect the motors and flow divider from cavitation (self destruction). But in order to reverse you have to close those check valves so that the flow goes back through the motors instead of around them (and thru the flow divider going forward). I have it setup to protect the motors and flow divider in both forward and reverse. It looks big on the schematic but it will be a compact little manifold circuit.

As is shown in the schematic there is already an orifice between the flow divider motors for the front and rear sections. This provides the limited slip function. I could include this feature in the anti-cav manifold if needed. I based the schematic symbol off the "D series" Haldex rotary flow divider: http://www.haldex.com/Global/Hydraulics/Product%20Catalogs/flow_div_1205.pdf

The Haldex product seems to be the closest to the right ratings, but looks like their biggest D series max flow is 14gpm per section. To get the max speed we planned, we need more like 20gpm per section. The pressure might be a bit low. I'm not sure. It says maximum inlet pressure 3000psi, maximum outlet pressure 4500 psi. I was planning the system to have a max of 4500psi as that is the max pressure of the motors. Not that we want to hit the max often. I can't say I totally understand how the outlet pressure can be higher than the in.