-
Thank you for visiting HeavyEquipmentForums.com! Our objective is to provide industry professionals a place to gather to exchange questions, answers and ideas. We welcome you to register using the "Register" icon at the top of the page. We'd appreciate any help you can offer in spreading the word of our new site. The more members that join, the bigger resource for all to enjoy. Thank you!
New Bobcats, No DPF, no DEF and a bobcat diesel engine. Thoughts and experiences?
- Thread starter Todd v.
- Start date
Looking over the Cat specs you are probably correct but I haven't found anything to confirm it. I notice on the 246D (radial lift) and 262D (vertical lift) they are rated @ 23gpm while the 242D is at 20. I will find out tomorrow from the salesman.
DirtCrawler
Well-Known Member
whats the cost to repair/replace a cat gear pump vs a piston pump?
durallymax
Senior Member
The pumps are positive displacement so the advertised flow is what they will produce regardless of pressure. The advertised pressure is what they can operate at safely.
To back that up, cat advertises their hydraulic HP for their machines, given that info yes they will put out max flow at the pressure advertised. Hydraulic power is a simple calculation. GPM*PSI/1714=HP. Now this does assume 100% efficiency which is never the case.
242D Standard: 20*3335/1714=39hp.
Now the technical side to understand it some or become more confused. First off, pumps produce flow not pressure, pressure is resistance to flow. The pumps in this situation are all positive displacement which means that regardless of pressure they will produce the same amount of flow. There is a limit to what they can operate at though (stall point) and that is why all systems have a relief valve. Gear pumps are what's used in these machines and gear pumps are fixed displacement, meaning they are always putting out the same amount of flow per revolution. Their operation is simple, gears of some sort meshing with each other and squeezing a fixed amount of fluid between them. So if its max flow is 20gpm @ 2,500RPM thats what it will be no matter what, if there is 100psi it still produces 20gpm, if theres 3300 it still produces 20gpm but remember the power calculation, its taking 38hp instead of only 1hp.
Piston pumps are the other pumps used in these machines and they are almost always variable displacement. In this case they are axial piston pumps and their output is controlled by a swash plate. Piston pumps operate by using pistons that move up and down to displace a certain amount of fluid. The amount of fluid is controlled by the swashplate. The angle of the swashplate controls how much the piston can move up and down. The old school drive controls are an example of this, your lever is simply moving the swashplate to increase/decrease the displacement or change the direction of the flow. EH controls just replace your lever with electronics. So what controls the displacement of the auxillary pump in this case? The load sensing control. This is where the big advantage comes in over the fixed gear pump. In simple terms the load sense control recieves pressure from the pump and pressure from the load. The LS control has an internal spring, lets say its set at 200psi so it works by keeping the pump output 200psi higher than the load pressure. If the load is 3000psi, the pump will be putting out 3200psi. But remember pressure is just resistance to flow, so what is the flow? Depends on what you are doing, thats whats great about variable displacement. If the tool needs 10gpm at 2,000psi, the pump will put out 10gpm. At idle the pump is only making 200psi because of the LS control's internal spring. Now when you push the button for flow in the machine and the auxillary spool valve opens, the fluid starts to flow to the tool but since you only have virtually no flow at 200psi it can't move the tool and thus pressure increases which the Load sense valve then sees and thus moves the swashplate for the pump to increase the displacement. It will not stop increasing the displacement of the pump until the pressure difference is back to 200psi or the pump is at max displacement which is the max angle the swash plate can be moved. It is very rare that the pressure and flow requirements are ever consistent with any work tool so the load sensing can really save power and fuel by only supplying what is actually needed. The last part of this system is the pressure compensation which is present in the Cat system and many load sensing variable displacement pumps. Pressure compensation de-strokes the pump when a preset pressure is hit, let's say 4,000psi. So if you are running a mulcher and stall it against something, the pump will destroke to 0gpm and thus you wont be wasting power and creating heat by going over relief. There is still a relief valve in the system, but set higher than this. This all obviously happens extremely fast and the end result is only using the power and fuel needed to accomplish the task at hand while reducing the overall heat production as well which increases efficiency and decreases cooling needs.
The above is just what is happening with the actual pumps, there are a lot of outside controls that influence things as well. You may be thinking, "if the gear pump is fixed displacement, why can I use a variable control thumbwheel?" That is where proportional controls come into play downstream of the pump. Another topic for another day.
Confused more yet?
The 242D just uses standard "High Flow" which is a gear pump. The 262D uses XPS High Flow which is a load sensing proportional control variable displacement piston pump. I presume what they are doing when you hit the high flow switch is allowing a larger swash plate angle (allowing more flow) and changing the proportional control setting (destroking the pump at a higher pressure). Exactly how, I do not know nor can I tell from the parts book.
So is it better than Bobcat? I don't know their system well enough to even make a comment on it. If they're using gear pumps does that mean its not load sensing and inferior? Not necessarily there are load sensing gear pumps and ways to have a load sensing system with a fixed displacement pump. Not always the most efficient or simple setups though.
durallymax
Senior Member
On a 262D, the prices I see for the difference between the standard flow gear pump and XPS piston pump is about 1,000 versus 1,900. Thats buying a complete new assembly.
Got all that. You can even add an additional factor to the theoretical hyd hp calculation to factor for inefficiency, typically 5%-10%. My primary point is that if you put a flow meter on a machine and dial up the pressure to 2000psi you will not be getting max flow through it. I have done this to numerous Bobcat, Case and Cat machines. The flow advertised on the spec sheet will be seen on a flow meter with no load on it. Once you increase the load the flow decreases.
DirtCrawler
Well-Known Member
So both the 242 and 262 are gear pumps, unless you get high flow in either then they are piston pumps ??
durallymax
Senior Member
No the only piston pump is XPS on 262. 242 high floe I'd still gear
Great explanation Durallymax, I like knowing the technical side of things. Of course how can you diagnose a problem if you don't understand how it's supposed to work?
The salesman told me they were all variable displacement piston pumps, not including the high flow machines, I went up to the Cat dealer yesterday. I know they don't always have the facts though and it's in none of the literature. I ended up favoring the 262D machine though and am trying to get one to demo next week. The price is really right in line with the Bobcat machines if not a little cheaper when you consider what you are getting.
Another point was brought up about how small the hydraulic fluid reservoir is in the Bobcat machines, seems like that fluid would have a hard life being cycled through so many times in a minute when you are running they hydro hard. Temperature fluctuations and all that included.
The salesman told me they were all variable displacement piston pumps, not including the high flow machines, I went up to the Cat dealer yesterday. I know they don't always have the facts though and it's in none of the literature. I ended up favoring the 262D machine though and am trying to get one to demo next week. The price is really right in line with the Bobcat machines if not a little cheaper when you consider what you are getting.
Another point was brought up about how small the hydraulic fluid reservoir is in the Bobcat machines, seems like that fluid would have a hard life being cycled through so many times in a minute when you are running they hydro hard. Temperature fluctuations and all that included.
durallymax
Senior Member
It's good to understand the theories but also to see how it actually translates to real life as well. As dig deep pointed out, nothing really puts out what it's advertised at but it should still be close. I'd be curious to know what the service specs show the ratings to be at when diagnosing an issue.
It'd be nice if all salesman had a technical knowledge of the machines but I can't blame them for not keeping up on everything. They can do what they want, the good ones sell more machines and make more money. The best ones don't necessarily know anything but know the right people to ask for answers before giving one.
I don't work for Cat or have much info on the machines other than a parts book for the 262D which states the pump is a gear pump unless the machine has XPS then it says it's a piston pump. The load sensing pressure compensated variable displacement part is from their sales material and based off what the XPS has been in the past. The 242D I assume is a gear pump because it still operates at the same pressure. Gear pumps have a much lower pressure limit than piston pumps. In the past they've used gear pumps on that size machine for high flow. I only have a book for a 242B3 and it shows a gear pump for high flow.
I wouldn't really get too worried about bobcats reservoir size. If the system keeps the fluid within its parameters it will not have any issues. The only thing a big reservoir does is give you more fluid to heat up. Once you get that fluid hot, having a bigger reservoir in this case is not going to help at all provided you are doing continuous work. The fluid is not going to cool off significantly by sitting in a tank in the belly. It's cooled by the cooler. True that bigger reservoirs can sometimes help a little but not much. The fluid has very little time to sit in the tank if you are running a high demand continuous operation tool. These machines should be designed to run continuous without issues. The only other advantages of a big reservoir is more reserve capacity for leaks, or if you need to fill something like a dump ram. Not something you would be doing with a skid steer, but something like a dump truck needs enough fluid capacity to fill up the ram.
I'm not a fluid engineer, but in theory you would think that 5 gallons of fluid is being used twice as much as 10. I don't see that as a big deal either, just potentially means a service interval that is shorter. Not a bad thing, so you change the fluid twice as often it's still the same amount of fluid to buy granted a bit more labor though but it's not very often you are changing it.
I really doubt Bobcats system is designed in a way that it would overheat or compromise the fluid. Keeping their coolers clean may be tougher though but that's a different topic.
It'd be nice if all salesman had a technical knowledge of the machines but I can't blame them for not keeping up on everything. They can do what they want, the good ones sell more machines and make more money. The best ones don't necessarily know anything but know the right people to ask for answers before giving one.
I don't work for Cat or have much info on the machines other than a parts book for the 262D which states the pump is a gear pump unless the machine has XPS then it says it's a piston pump. The load sensing pressure compensated variable displacement part is from their sales material and based off what the XPS has been in the past. The 242D I assume is a gear pump because it still operates at the same pressure. Gear pumps have a much lower pressure limit than piston pumps. In the past they've used gear pumps on that size machine for high flow. I only have a book for a 242B3 and it shows a gear pump for high flow.
I wouldn't really get too worried about bobcats reservoir size. If the system keeps the fluid within its parameters it will not have any issues. The only thing a big reservoir does is give you more fluid to heat up. Once you get that fluid hot, having a bigger reservoir in this case is not going to help at all provided you are doing continuous work. The fluid is not going to cool off significantly by sitting in a tank in the belly. It's cooled by the cooler. True that bigger reservoirs can sometimes help a little but not much. The fluid has very little time to sit in the tank if you are running a high demand continuous operation tool. These machines should be designed to run continuous without issues. The only other advantages of a big reservoir is more reserve capacity for leaks, or if you need to fill something like a dump ram. Not something you would be doing with a skid steer, but something like a dump truck needs enough fluid capacity to fill up the ram.
I'm not a fluid engineer, but in theory you would think that 5 gallons of fluid is being used twice as much as 10. I don't see that as a big deal either, just potentially means a service interval that is shorter. Not a bad thing, so you change the fluid twice as often it's still the same amount of fluid to buy granted a bit more labor though but it's not very often you are changing it.
I really doubt Bobcats system is designed in a way that it would overheat or compromise the fluid. Keeping their coolers clean may be tougher though but that's a different topic.
JCBiron
Well-Known Member
Don't confuse combustion temperature with coolant or operating temperature. The cooling systems on the machines are designed to handle the heat generated by higher combustion temps. In fact, in some cases, the cooling systems on DPF machines are even larger due to the fact that they have to cool down all that exhaust gas before recirculating it back into the intake. Seems arse-backwards considering the whole concept of a DPF system is to lower combustion temps to control NOx gas, as Durallymax very effectively explained.
Keep in mind also that most of these OEM's that are just now introducing these technologies on smaller HP engines (i.e. Skids and CTL's) have been toying with the tech for several years on larger machines. Not to mention the On-road industry's experience as well. The tech is still somewhat new (relatively speaking), but some of the on-road suppliers are approaching 10 years of experience.
I'm a fan of SCR, and I think if nothing else, the fact that a DPF-only system cannot meet regulations on its own proves (somewhat) that it is the better solution for the problem (emissions control). Just my thoughts.
Well I ended up ordering a Bobcat S630 with A/C and hi-flow and the good instrumentation. The Cat was definitely smoother and quieter but when everything was said and done I didn't have a strong lean either way and Bobcat did a much better job selling the machine and giving me a good deal. And they will have it to me in 30 days or so where the Cat would have been 3 months I was told.