Weight on Rear of PT

BobRip · May 25, 2007

J_J said:
Bob,

Have you considered the fact that Pt probably came up with the lift capacity taking into account the tipping point, and the load on the wheel motors?Sure, more weight on the back will allow you to lift more, but will your front wheel motors stand up to the additional shaft load that is generated by adding extra weight. Reversing wheels in this situation, I think not, but to each, do what you have to do.

J_J, when I purchase the PT, I was told that it would lift 800 pounds, and that was the load not including the attachment. I used my forks to pick up a stone that weighted 819 pounds (as weighted by the supplier of the stone). The PT would barely lift this with a person standing on the bumper. I think all my weights are doing is bring the PT upto it's rated lift capacity and that the hydralic relief valve is what controls the limit and protects the machine. It might be good to call PT on this. Also even with the 120 pounds on the back, it will still not lift 800 pounds. It takes about 180 to do that.

Since I don't lift 800 pounds every day, I doubt this will really cause a problem. When the first wheel motor goes, I am putting in stronger ones anyway.

Barryh · May 26, 2007

BobRip said:
J_J, when I purchase the PT, I was told that it would lift 800 pounds, and that was the load not including the attachment. I used my forks to pick up a stone that weighted 819 pounds (as weighted by the supplier of the stone). The PT would barely lift this with a person standing on the bumper. I think all my weights are doing is bring the PT upto it's rated lift capacity and that the hydralic relief valve is what controls the limit and protects the machine. It might be good to call PT on this. Also even with the 120 pounds on the back, it will still not lift 800 pounds. It takes about 180 to do that.

Since I don't lift 800 pounds every day, I doubt this will really cause a problem. When the first wheel motor goes, I am putting in stronger ones anyway.

Different opinions and point of views for different personal reasons. That’s what makes this forum great. Even though we may not all agree it helps make informed decisions. Weight on the back is something I would take a chance on as I have noted for me, it has been needed in many situations. This may not be as much of an issue for some, who may have different models that weight more, and can lift more without the need for a counter balanced weight in the back.

I have also needed more stability when moving things around my uneven off camber yard. I’m thinking the best of both worlds is probably Kent’s idea of loading up the tires for less all around stress on the PT with some increased lifting, and to help stabilize it somewhat. When needed, Bobs idea of extra weight in the back that can be added or taken off at will as not to break the back of the PT.

My only concern might be the added pressure on the articulating joint mounts in the middle of the machine and also as J.J. mentioned, the wheel motor shafts which I think are under tremendous strain when just lifting overly heavy objects at the rated loads.

Reversed wheels in extreme situations when safety is a (real issue), if you traverse a lot of really steep hills, where you have real tendency to roll as in Kent’s property. It may be well worth the extra expense and shortened life of the wheel motor, verses injury to your body. For me probably not worth the expense.

KentT · May 26, 2007

FYI. The specs for the White RS wheel motors, shows on pg 14 that the motors are rated for at least 1400 lbs of sideload each at approximately 185- 190 RPM.

The acceptable sideload goes down as a factor of RPM going up, and unfortunately the specs do not show the maximum sideload of these wheel motors at very low RPM -- typically under 100RPM in normal use.

However, the curve of the graph's parabola is flattening from about 1100 pounds to 1400, the max value charted, which would certainly indicate that 1500 lbs or more is acceptable at the lower operating RPMs of these wheel motors (about 144RPM max for the White RS 12.5ci motors that Bob and I both have, as far as I can tell). Note that this measurement is taken at the center of the Woodruff key on the shaft, and would be reduced by changing the offset of the wheels so that there is more distance on the outside than the inside (such as I have done by reversing mine).

Bottom line is that I think you can readily handle 3000 lbs or more load on the TWO front wheels without worry, with the tires/wheels in the normal configuration, operating on level ground. Note this introductory info from White's Allowable Side Load Chart:

Operating conditions within the shaded area will maintain acceptable oil film lubrication with recommended fluids. Operating conditions outside the shaded areas are susceptible to motor failure due to oil starvation and/or excessive heat generation. Fluids with low lubricity or low viscosity may require the maximum load and speed ratings to be derated to provide acceptable motor life and performance.

White's performance charts for those RS motors show testing done at 129 defrees F, with an oil viscosity of 213 SUS. Here's a viscosity conversion chart that might help interpret that SUS rating:

Viscosity Charts

Point is that increasing the oil viscosity also increases the ability to handle sideloads...

One final note -- the newer-style wheel motors that PT uses now are what I call "short, stubby" ones (the manufacturers call them "compact" style). They have 1.25" shafts instead of 1" shafts like these old White motors, and they ALSO have tapered roller bearings instead of just thrust bearings (like the White RS I have or the Char-Lynn S series that I just ordered). Consequently, the newer style motors with 1.25" shafts can handle MUCH higher sideloads. The layout of the graph in the specs for the White CE motors used on the new "higher-torque PT-425s" shows at least 3,000 lbs of allowable sideload EACH at the center of the Woodruff key -- based upon a 2,000 hour life expectancy (MBTF) at 100 RPM. Perhaps as much as double the load of the older style motors...

Bottom line is that I don't think you really need to be overly concerned about overloading the bearings in the wheel motors, especially on the newer PTs. On the older ones, like mine, 3000 lbs of weight on the front is within spec, as long as you don't reverse the wheels. With reversed wheels that would be less -- and I don't have the data to predict how much, or what that value would be. However, I don't foresee the likelihood of having more than 2200 lbs or so on the front wheels -- the total of 800 lbs lift capacity and the 1400 lbs of the PT itself -- and that only in a "pucker" situation where all of the weight of the PT is on the front wheels and the rear is in the air....

I realize I'm pushing the limits, but I also hope to be pushing the limits of the Kohler's oil system, i.e. I hope to be able to operate on 25 degree slopes with my new wheel motors. (However, I don't see having 800 lbs in the bucket doing so, unless I'm pointed uphill.) Those new Char-Lynn S series motors also show a similar 1400 lb allowable sideload measured at the center of the Woodruff key at up to 200 RPM, similar to the White RS. However, Char-Lynn's performance specs are measured at a much lower 120 SUS viscosity...

Just some more data to chew on...

Barryh · May 26, 2007

KentT said:
FYI. The specs for the White RS wheel motors, shows on pg 14 that the motors are rated for at least 1400 lbs of sideload each at approximately 185- 190 RPM.

The acceptable sideload goes down as a factor of RPM going up, and unfortunately the specs do not show the maximum sideload of these wheel motors at very low RPM -- typically under 100RPM in normal use.

However, the curve of the graph's parabola is flattening from about 1100 pounds to 1400, the max value charted, which would certainly indicate that 1500 lbs or more is acceptable at the lower operating RPMs of these wheel motors (about 144RPM max for the White RS 12.5ci motors that Bob and I both have, as far as I can tell). Note that this measurement is taken at the center of the Woodruff key on the shaft, and would be reduced by changing the offset of the wheels so that there is more distance on the outside than the inside (such as I have done by reversing mine).

Bottom line is that I think you can readily handle 3000 lbs or more load on the TWO front wheels without worry, with the tires/wheels in the normal configuration, operating on level ground. Note this introductory info from White's Allowable Side Load Chart:

Operating conditions within the shaded area will maintain acceptable oil film lubrication with recommended fluids. Operating conditions outside the shaded areas are susceptible to motor failure due to oil starvation and/or excessive heat generation. Fluids with low lubricity or low viscosity may require the maximum load and speed ratings to be derated to provide acceptable motor life and performance.

White's performance charts for those RS motors show testing done at 129 defrees F, with an oil viscosity of 213 SUS. Here's a viscosity conversion chart that might help interpret that SUS rating:

Viscosity Charts

Point is that increasing the oil viscosity also increases the ability to handle sideloads...

One final note -- the newer-style wheel motors that PT uses now are what I call "short, stubby" ones (the manufacturers call them "compact" style). They have 1.25" shafts instead of 1" shafts like these old White motors, and they ALSO have tapered roller bearings instead of just thrust bearings (like the White RS I have or the Char-Lynn S series that I just ordered). Consequently, the newer style motors with 1.25" shafts can handle MUCH higher sideloads. The layout of the graph in the specs for the White CE motors used on the new "higher-torque PT-425s" shows at least 3,000 lbs of allowable sideload EACH at the center of the Woodruff key -- based upon a 2,000 hour life expectancy (MBTF) at 100 RPM. Perhaps as much as double the load of the older style motors...

Bottom line is that I don't think you really need to be overly concerned about overloading the bearings in the wheel motors, especially on the newer PTs. On the older ones, like mine, 3000 lbs of weight on the front is within spec, as long as you don't reverse the wheels. With reversed wheels that would be less -- and I don't have the data to predict how much, or what that value would be. However, I don't foresee the likelihood of having more than 2200 lbs or so on the front wheels -- the total of 800 lbs lift capacity and the 1400 lbs of the PT itself -- and that only in a "pucker" situation where all of the weight of the PT is on the front wheels and the rear is in the air....

I realize I'm pushing the limits, but I also hope to be pushing the limits of the Kohler's oil system, i.e. I hope to be able to operate on 25 degree slopes with my new wheel motors. (However, I don't see having 800 lbs in the bucket doing so, unless I'm pointed uphill.) Those new Char-Lynn S series motors also show a similar 1400 lb allowable sideload measured at the center of the Woodruff key at up to 200 RPM, similar to the White RS. However, Char-Lynn's performance specs are measured at a much lower 120 SUS viscosity...

Just some more data to chew on...