Hydraulic Motors & Negative Braking - Small Tracked Vehicle

[Pete1961]

I am looking at adding braking to a little 700Kg kit tracked excavator. At present it is limited to slopes of 10 degrees up & down hill. If I use a georotor motor with negative brake (fail safe spring applied, braking removed by pressure via shuttle valve) & a dedicated motor spool directional valve or DCV, then what are the overrunning circuit considerations if any, I need to guard against? The design at present has the motor controlled by a cylinder / ram type DCV spool valve & no brake or additional valves whatsoever

The motor type DCV spool has both motor lines open to neutral (tank / atmospheric pressure) when at rest & also the return line is open to the tank when motor is running. A shuttle valve connects whichever motor line is selected, providing positive pressure to the brake remove / hold off pistons. 200 psi is needed in this selected line to start to lift the brake off, is totally off at 400 psi
Thus I cannot see how the two common bugbears of vacuum cavitation developing in the feed line, or overpressure spikes in the return line can ever occur. The brake will be on without the 200 psi positive pressure, so no rotation of the motor, i.e. it cannot become a pump. Also there is no chance of a pressure spike if an operator drops the control valve. I have some old 70's JCB tech handbooks, these show no additional valves beyond the shuttle & the motor type DCV

Am I missing anything here? I know if a motor it is fed by a cylinder (ram) spool then both above mentioned problems occur on overrunning, the anti vacuum & cross line relief valves then needed to protect the motor, in doing their job create a situation where the vehicle can then run away, hence a mechanically applied back up brake is also needed. This is called positive braking & can be seen on the Struck Corp tractor for instance. Negative braking is much more common on hydraulic track vehicle drives, hence the motors or additional brake add on parts for same. Thanks for any help in this.

 

[Pete1961]

Sorry, this could read clearer with hindsight. At present there are georotor hyd track motors controlled by cylinder DCV's & no brake at all. Neither is there any anti cavitation circuitry, nor any anti pressure spike cross line relief valves . Hence the 10 degree slope limit. Machine can be seen here.

http://hho.fr/mini-pelle-materiel-en.html

I am in talks with the designer / Kit maker about making a long & wide track version to work on slopes. The second paragraph concerns the proposal for Negative fail safe braking - using spring applied add on brakes or a motor with this & the shuttle valve integrated into it.
I really wish to know if I need to add anything extra between a 'motor type' DCV spool & the braked motor

There is an added problem of side loading of the motor shaft (this carries the track driving sprocket) This I propose to get round by having a separate stub shaft in a plain bearing carrying the track sprocket, connected axially to the braked motor via a coupler like this

http://www.gmspolymer.co.uk/flexible-couplings.htm

The stub shaft & motor shaft both have flanges with pins protruding (the flanges facing one another) The hardy Spicer type coupler connects the two flanges & transmits drive
There is room for this because my chassis is wider than normal. This obviates the need for any chain drive, keeps the small track sprocket, & in short messes with the original design as little as possible. Making the permanently wider chassis is already agreed by the kit maker, indeed proposed by him. It is only the braking problem that is to be solved, negative fail safe is preferred but positive manual braking a fall back position. Thanks for any help, Pete

 

[HPost]

Back driving hydraulic motors needs about 80% of their full torque to get them started.

If you have a brake on the motor, the forces will have to overcome the brake friction for each motor and the each of the motors on the machine.

If the motors do start to turn, that is the point at which oil needs to be allowed to flow in. Otherwise the motor inlet will see low pressure and it will start to boil the oil at the inlet. Adding motor spools will help that.

You know that you need to avoid side loading the motors

I can’t see that you need any help here, which is probably the reason that opinions on this have not been forthcoming. There is not much to say really.

 

[Jacc]

Brakes of this type are typically meant to be static brakes only, dynamic braking should be handled by some sort of counterbalance valve.

There are special spool type counterbalance valves for track drives, very different from ordinary counterbalance valves but I doubt they go that small.
Since your machine is so small maybe a ordinary counterbalance valve will do the job. Some more heat generation but that may not be a problem.
For some orbit motors there are flangable counterbalance valve blocks with brake release port.