marrt
Platinum Member
Ok, let's talk UTF once again. At least for the Bondioli & Pavesi NT10 pump used in the 425. This pump is optimized for a hydraulic viscosity range of 15 - 35 cSt (mm2/s). Of course, you cannot talk about viscosity without talking about the associated temperature. Most viscosity ratings are given at 40C and 100C. Our "normal" operating temperature is probably something closer to 150F (that's a guess... I need to measure it) or roughly 65C. So, for simplicity, let's choose a midpoint between 40C and 100C, or 70C. Also, the change in viscosity between two different temperatures is not perfectly linear, but probably close enough for this exercise.
Now, take a look at this table (Viscosity Charts - Bob is the Oil Guy - Bob is the Oil Guy). Let's assume we are going for the mid-point range for the pump, or 25 cSt. At 70C, the target cSt is 17.5 for a SAE right between 40 weight and 50 weight engine oil. Even the maximum recommendation of 35 cSt is well within range of 50 weight engine oil.
What does all this mean?
1. The guys at Power Trac probably know what they are doing specifying a multi-weight engine oil. If this was an application in a temperature controlled factory, then you could easily specify a fixed weight hydraulic oil. However, because Power Trac's must be able to function in low temperature conditions, a multi-weight oil is required.
2. Using a standard ISO hydraulic oil or even a Universal Tractor Fluid hydraulic oil (with the usual narrow viscosity range of 10w-20 or so) will probably not work well for a Power Trac. These oils would be too thin to stay with the optimized viscosity range for the NT10 when the tractor is "warmed up" while still being able to allow the tractor to start at freezing temps. This is unfortunate because these oils have many superior characteristics when used in a hydraulic application.
3. Changing to wide range synthetic motor oil such as a 10w-50 would appear to offer the best option to hit the target viscosity range for the NT10. However, the wider the viscosity range, the poorer the sheer characteristics over time. Therefore, Power Trac's choice of 10w-40 makes a lot of sense for dino oils.
Now, take a look at this table (Viscosity Charts - Bob is the Oil Guy - Bob is the Oil Guy). Let's assume we are going for the mid-point range for the pump, or 25 cSt. At 70C, the target cSt is 17.5 for a SAE right between 40 weight and 50 weight engine oil. Even the maximum recommendation of 35 cSt is well within range of 50 weight engine oil.
What does all this mean?
1. The guys at Power Trac probably know what they are doing specifying a multi-weight engine oil. If this was an application in a temperature controlled factory, then you could easily specify a fixed weight hydraulic oil. However, because Power Trac's must be able to function in low temperature conditions, a multi-weight oil is required.
2. Using a standard ISO hydraulic oil or even a Universal Tractor Fluid hydraulic oil (with the usual narrow viscosity range of 10w-20 or so) will probably not work well for a Power Trac. These oils would be too thin to stay with the optimized viscosity range for the NT10 when the tractor is "warmed up" while still being able to allow the tractor to start at freezing temps. This is unfortunate because these oils have many superior characteristics when used in a hydraulic application.
3. Changing to wide range synthetic motor oil such as a 10w-50 would appear to offer the best option to hit the target viscosity range for the NT10. However, the wider the viscosity range, the poorer the sheer characteristics over time. Therefore, Power Trac's choice of 10w-40 makes a lot of sense for dino oils.