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Re: The "Friction Wheel" versus The Hydrostatic Transmission drive debate
#1 Apr 27, 2014 1:05 pm |
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Well, since the thread was resurrected I wonder if a couple of misunderstandings of the friction drive might be up for discussion.
I too have had no problems with the friction drive on my 1982 Toro 724. I inherited it from my brother when he moved to AUS. It's spent its whole working life in West Michigan near Muskegon. I'd say we get snow in all varieties and have had some real doozies in addition, of course, to many sparse years in between. But it's the original rubber drive in there. This clutch has been maintained well and inspected yearly and looks to me like it was easily intended to last the life of the machine. My blower is in near-new condition and well past its likely intended lifespan. Clutch amazingly shows little signs of wear. I've greased, oiled, inspected, replaced parts 95 percent of the time with off-shelf items from belt and bearing suppliers (not 2x to 3x more expensive OEM), repainted and tig welded when needed. It's still in such good shape that last fall I replaced the still fine but tiring 7hp Tecumseh with an 11hp OHV Techumseh for 150 dollars shipped to the door.
The clutch's robustness and power to the wheels have always defied intuition for me. A simple rubber wheel held against a smooth steel disc driving a heavy machine into the snow? There must be more to this. So I stared at it one evening when I was in there greasing all the shaft bushings. Sorry if this is actually common knowledge but I saw no reference to the real mechanics of this described in the thread.
The spring, unlike as described here and generally understood, does not hold the rubber to the disc. In fact it pulls in the other direction! Hmm. If the spring is too tight (not likely is using an OEM) it will only serve to make engaging the drive at the handle grip more difficult, but will have zero effect on the pressure/engagement of the rubber against the disc. The spring in question only serves to DISengage the drive, not hold the rubber to the disc. So what is holding the rubber to the disc? Not just the handlebar grip since that would make it very difficult to hold such strong engagement by the hand alone and would be tiresome. The answer is a pretty elegant camming action and obvious if you look closely.
My Toro rotates the rubber drive wheel against the disc. Some other designs bring the steel drive disc against the rubber wheel. Both work on a similar principle of jambing the faces together with a camming action.
There must be better engineering terms to describe this. Sorry I'm not an engineer. The rubber clad wheel is cammed about its axis. I'm not saying, of course, that the rubber clad wheel rotates eccentrically.... only secondarily about its axis. When that assembly is rotated toward the steel drive disc the rubber engages the disc at a point just above the centerline shortest distance between the disc face and the rubber clad wheels' eccentric axis. At this point what you are doing at the handlegrip is pulling against the clutch drive spring. The clutch drive spring is keeping the rubber wheel away from the disc, not against it. Once you've overcome the spring resistance with the handlegrip to bring the rubber to touch the spinning drive disc it grabs the rubber wheel, and using the rubber wheels' resistance (since it's mechanically static and linked to the wheels) it drives the clutching faces a few degrees beyond the shortest distance between them determined by the rubber wheels' axis. This jambs the rubber wheel against the steel disc with pressure far greater than could be accomplished with a spring alone. Once in this position the only way to unjamb the rubber drive from disc is to release the handlebar grip and allow the strong clutch spring to pull the rubber back and out of its jamb against the disc.
Anyone noticed that when this spring breaks or goes waaay too weak it does not result in a no-drive or weak drive situation? If the spring breaks the clutch will remain engaged in the gear it was in and you can't stop the beast. There is no longer any spring to pull the rubber wheel away from the drive disc. Another indication that this jambing/camming action is at work is that reverse is significantly less powerful than the forward settings. The jambing design is, in reverse, working opposite of its intended design. In reverse the only pressure applied to clutching faces is what you provide with the handlegrip since the disc is now rotating up against the rubber instead of downward and no jambing takes place. The handlegrip linkage itself is levered at a point only a degree or so before it would jamb (and then render the clutch return spring useless in pulling it back) so it provides sufficient ease on the hand when holding it in reverse for a short while.
If the spring has nothing to do with the pressure of the rubber clad wheel against the drive disc I wonder what else might be at play when some notice that the drive is slipping. It's easy to see how grease might be a culprit if it's not applied carefully. Once slippage is induced over a period of time glazing would occur. Nothing to do but sand it smooth and try again with a clean setup. I don't believe that the steel face can become "too smooth", The smoother the better as long as it remains dry and oil free. Smooth = less wear on the rubber. In a well 'jambed' state a rough surface should provide no better friction. Rubber formulation and local climate are probably the worst factors. living near a corrosive salty ocean or running the blower on heavily salted surfaces would affect the life of all components, metal or rubber. A poorly forumulated OEM rubber would crack and break down much faster than a superior one.
What might be interesting (just a little) is the relative efficiency of the friction vs hydrostatic. No doubt the hydrostatic is higher, and (i think) usually incorporates some form of differential to the drive wheels which must be a very nice addition for ease of use. All totalled I would personally opt for the friction drive again if given the choice between two identical models (just for sake of comparison.... i doubt that choice exists between two otherwise alike blowers). The friction drive has proven itself to me, and I'm confident enough of my mechanical skills to fix it cheaply if it fails for whatever reason (unlike hydrostatic which would likely be an expensive fix or total replacement),
Sorry about all that if it's confusing, or worse, foolishly overstating the obvious for most here. An engineer might have explained that in one paragraph.
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