Powershift Mysteries Revealed
The Yanmar Powershift is surrounded by confusion. Is it a hydrostat? How does it work? Is it reliable? How do I use it? We hope this will help clear up some of that confusion.
First of all... the Powershift is NOT a hydrostat. A powershift DOES use hydraulic pressure to engage the clutch packs but the actual power to the transmission comes from a mechanical link to the engine like in a manual transmission.
In a hydrostat, (warning: extreme over-simplification ahead) the engine drives a pump that pumps oil to a hydraulic motor that turns the transmission. This makes the hydrostat infinitely adjustable. You can pretty much go any speed you want with the engine at 2,000 RPM, for example. You just push the hydrostat pedal harder (some have a lever- but most are pedals now) to go faster or pull the pedal up to go backwards and the engine stays running at the same speed. It is very handy for repetitive back & forth tasks but it comes at a cost: waste. Hydrostats are simply not very efficient. One study found that they were around 20% less efficient than a mechanical transmission (takes 20% (+/-) more horsepower to do the same work). Turning a pump which pumps oil through a valve to drive a motor which drives the transmission wastes a lot of power. The pump wastes power, the valve wastes power, the motor wastes power, etc. Hydrostats are typically not recommended any time you need to pull things a lot - like a plow, or disc, or frequent heavy box blading).
In contrast, the Yanmar PowerShift is almost just as efficient as a manual transmission and most of the engine's power makes it to the wheels.
How does it work? The PowerShift portion of the transmission is controlled by a shifter, usually between your legs or on the dash. The shifter operates a valve that directs oil to certain sets of clutches inside the transmission. These clutches lock together (under extreme pressure to avoid any slippage) and cause the transmission to turn different sets of gears moving the tractor at different speeds. The oil pressure comes from a PowerShift pump located inside the bellhousing. It does not use the same pump as the three point or the loader. It does, however, share the same oil reservoir.
How do you use it? The Yanmar PowerShift has a manual range gear shifter. You need to push in the clutch to change these gears. Once the range gear is selected (usually a Hi, Med, & Low range) you can shift the PowerShift without using the clutch. You can select reverse, neutral, and 1,2,3, and on some models, 4th gear all while under a load. This is really nice for loader work where you are changing directions frequently. If you shift at a high engine speed without using the clutch you will get a very firm shift- enough to startle you if you are not paying attention and can even be dangerous in the wrong conditions. If you shift at a low rpm (as recommended) without the clutch- the shift will be firm but not too aggressive. So, should I use the clutch or not? Shifting without using the clutch will save wear & tear on your Yanmar's clutch but you can always use the clutch at any time if you want. You must use the clutch any time very slight movements are required (like backing up to hook up a mower).
The disadvantage of a powershift (or any mechanical transmission) over a hydrostat is that at any set RPM your travel speed is limited to however many gears you have. If you have 3 range gears and 3 forward powershift gears, for example, and you want to mow with the engine at a fixed 2,200 RPM: then you only have 9 options for forward travel speed. You can increase or reduce the engine RPM but then that will effect how your mower is performing. You can get into a situation where one gear is slightly too slow for your liking and the next gear is too fast for the conditions (too bumpy, sloped, etc). This is the situation where a hydrostat's infinite speed is superior.
Is it reliable? The Yanmar tractor PowerShift unit is extremely reliable. In all of the years we have sold parts for these tractors we have only sold 1 pump and 2 sets of clutch packs. Both of those were because the customer let the transmission run out of oil and let the clutches slip. If you maintain the oil level & change the oil as recommended you will never need to work on the PowerShift unit.
If you have any other questions please feel free to email us.