Downpressure

[exploder97]

I have a ST90 and im wondering if there is a way to adjust the downpressure, It seems to me that it really isnt putting much pressure on the blade, and i iknow i still have room for the lift cylinder to move around. Just wondering if anyone can help me here, It seems to be when activated it only really lifts the plow frame up a bit and i mean a tiny amount, and not really takes away pressure on the frame vehicle. Please advise me if this is something i can adjust. Thanks

Chris

[seville009]

I don't have the answer to your question re: adjusting, but I have the MT90 and I "tested" the downpressure when backdragging last year (my first year with it) by plowing with it off and then on. There was a noticable improvement in backdragging with the down pressure on.

When I engage the downpressure, the front of my truck does noticably lift up a bit.

[wyldman]

It is not adjustable that I know of.There is a pressure limit switch on the aux valve body just behind the reservoir.They may use different cut off levels (switch calibration) for different models,but I'm not sure.Check with your dealer to find out.Maybe the switch is weak,and cutting out early.

I've seen lots of them that the downpressure leaks down,but usually it will come back on,and will cycle back and forth.Leaky lift cyl is usually the culprit.

What you are seeing may be perfectly normal.It's not really supposed to lift the truck that much.Does it perform OK ? You should notice a difference with it on.You can use it for both backdragging and regular plowing when needed.

It should not bottom out the lift cylinder when it is on.It will extend some,and then stop when it reaches the limit pressure.

[bud16415]

Guys

One thing I discovered in building my own down pressure system and something no one ever thinks much about and that is the reaction of the device into the sprung weight on the front of the truck. It can only push as hard as what it is pushing against. So the heaver the truck the less raise you will see when bearing down on the road. On this type system and correct me if I’m wrong because I have never really studied one in detail. But the float you get well plowing while down pressure is being applied is the float you get from the front suspension. What really happens when you ride over a dip or bump is the cylinder acts as a fixed length link and reacts into the suspension until a point when the suspension pushes back with a force higher than the relief valve is set to and then the system dumps the pressure the amount of real float is limited by the type of truck you are using and how stiff and heavy the front end is.

As to the question though that valve could be fixed or adjustable that I don’t know. And if you remember it having more in the past my guess would be like Chris said to look at that first. I’m sure the dealer has a way to stick a gage in line someplace and watch to see what it pops at. They may set them up differently depending on what truck its mounted on also.

Good luck and let us know what you find. And also welcome to LTS.

Bud

[wyldman]

Bud - the downpressure stays constant,even as the plow floats and reacts to terrain changes.It will dump excess pressure off if needed as the plow is forced up,and then the pump motor will come back on if the pressure drops as the plow drops to follow the ground.

[bud16415]

This is from the snoway home page
http://www.snoway.com/adv_downpressure.cfm

Q. What if I hit an obstacle with the DPSystem on? Won't it increase the force of the hit?
A. No. When the DPSystem is on, it will sense an increase in pressure when an object is hit and open a relief valve, causing the blade to "float." When the pressure decreases, the valve will close.

I just looked that back up because I thought I remember reading that some time ago.
What I think they do is say just for talking purposes is when DPS is selected the system pumps the system up to 1000 psi and turns off they must use a PO check valve or something like that and also monitor the pressure in the cylinder (maybe) the relief valve would be Tee into the circuit and set to say 1500 psi. so there is a range that it tries to hold the DPS within. When within that range I think (and I could be wrong because I’m just guessing again) the system is locked up and floats with the front suspension.

The other method and I would be surprised if they did this would be to use a servo valve and constantly monitor the system pressure and adjust to it. It doesn’t sound like that is what they do and that is very hard to do under stable controlled conditions. The problem is kind of a hysteresis type thing almost. The time to react lags the actual need and just as you adjust the plow has moved ahead to a condition that requires the opposite signal to the valve and it never quite gets the job done.

I really think they are doing just what they say in their Q&A section a simple relief valve dumping pressure and then re closing and pumping the system back up again. And for that to work without constantly acting they have to have a high low set points.

Do you know if when in DPS the pump runs continually so that it will be ready to re pressurize the system or is there an accumulator? Otherwise there will be the time lag for the pump to come to pressure.

If anyone out there has a diagram to post it would be great to see. I didn’t see anything on their public page but they most likely have a service page for dealers.

Those same problems were the same reason I used the springs to hold down pressure on my plow. First being they react instantly to changes second they store the energy instead of reapplying it and third they allow the plow to float under the pressure they are exerting not the force of the trucks suspension. Lastly they don’t have that rapid point of protecting the cylinder by depressurizing rather they work over a greater range of travel.

Bud

[wyldman]

The excess pressure is dumped via a relief valve.If the pressure drops,the pressure switch (electrical) activates the motor,and it runs for a second to restore the downpressure.No accumulator involved.

It is actually quite quick,maybe a second tops to restore the downpressure.

[bud16415]

Thanks for the info Chris

At 10 MPH (no one plows faster than that do they?) if the relief valve dumps drops pressure sees that the high pressure situation has passed (still not sure how it knows to come back on?) pump kicks back on and restores DPS and it does that in say one second the truck will have covered 15 feet of ground. During that 15 feet a lot of terrain changes could have taken place. As in a dip in the road or the crest of a hill. In both cases the front suspension spring rate is what will decide when the system triggers off again. If the truck is light in the front the reaction lifting the front will allow the blade to stay under DPS longer and ride out a high point in the road. IMO
Now in the case of a very flat surface being plowed I see no problem. And the piping must be very large on the return side of the relief valve to get the pressure out fast enough to prevent damage. If the truck moves 15 feet in one second how short of a time do you have during contacting a manhole cover and the blade trip moving the edge backwards say 2 inches. That’s really the time the system has to react.

The guys that have them out their do you have any stories to tell about hitting obstructions while in DPS?

Bud

[wyldman]

Plowing with the DPS on is really no different.You don't really notice it at all,except it scrapes a lot better,and trips less in heavy snow.

When hitting an obstacle,it trips like anything else.

I'm not exactly sure how fast it reacts,but it's fast enough it never causes any problem.I've used it plowing out steep docks,and entrances without a problem.The pressure switch is what turns the pump on when needed.

[Robo]

On mine if backdragging a driveway and the truck starts down the approach while the blade is still "on top" of the driveway I don't notice the pump coming on as the blade is allowed to lift (pressure switch overloads and opens). To me it appears the pressure switch only lets off enough pressure to relieve itself while still maintaining the DP. If you go through a sharp trough or something like that then the pump comes on in pulses its way back to the proper DP. I have never really paid a bunch of attention as to how it actually works :confused: (mainly because I am not trying to build one). Don't know if this adds anything to the discussion or not.

I will say there are 2 things that suck about snoway IMO the mount and the stock headlights.

[landscaperpro]

Well, I run three of them. The down pressure as is being said, adjusts itself to the terrain consistantly. I have 2 9'1" v's and one 7'6" straight job, and regardless of the plow, they all work the same. Incidently, one thing you may want to check is to ensure that your frame is tight to the truck. IMO they made some drastic steps in improving the design of the frame this years, did away with the center pin, and now the head gears actually locks into cups at the push pins, and is then securely pinned up in the light towers. Run them on everything from a

I have noticed that extending the lights out further on the head gear significantly improves the visibility.