What exactly is over/under?

I'll preface this with saying, I've only dealt with a few small tractors. All but one with "conventional" transmissions. I did work a bit with a Ford that had something called "select-o-speed" or something like that which to me seemed kind of like an automatic tranny but with manual range selection.

I keep seeing posts about over/under or hydra shift or similar. I'd like to know exactly what these are. I'm only guessing about over/under, but I get the idea that it uses a "conventional" transmission and has some kind of under drive ratio and an over drive ratio available for each "conventional" ratio.

Is this so one can speed up in easy going or slow down in heavy work on the fly? Anyone care to enlighten me?

Nt sure if your old enough to remember the old over drive units one could install on cars and truck back I nthe day but the over under is pretty much just that, In over drive you have the say 4 gear in an over drive set up and then the normal gears

Hydra-power(2 speed) and Hydraul shift (3
speed) can/should be shifted on the fly.

Oliver 500, The Hydraul Shift is a three speed auxiliary unit interposed between engine clutch and transmission and provides on-the-go shifting in over, under, or direct drive. The auxiliary unit is self contained and includes its own reservoir. The Hydra-Power is also an auxiliary unit between engine clutch and transmission that provides direct drive and underdrive. First picture is Hydra- power drive out of a 1650. Second pic is the oil cooler for a 1555 hydra-power and third picture is a Hydra-Power out of my 1555. The 1650 unit I have had coolant lines that went back to unit. A chain coupler was used to hook Hydra - Power to splined shaft coming out of rear end. Last pic is a 1650 rear end and they could also have a Creeper Drive or
Reverse-O-Torc drive unit. The Reverse-O-Torc is a forward reversing auxiliary unit like most modern back hoes have. The first sentences are from my I&T manual.

Thanks all for the answers so far.

Internally, are they hydraulically or mechanically clutched planetary gears then?

Oliver 500, From the manual again. Drive selection is accomplished by engaging or disengaging multiple disc clutches and the operation of a planetary gear set and overrunning (Sprag ) clutch. The pressurized oil used to operate clutches, as well as oil for lubrication is furnished by a gear type pump mounted on inner side of housing cover. In overdrive position, oil pressure engages overdrive(rear) clutch and power flow goes through countershaft which drives the planet gear carrier. Overdrive clutch holds sun gear shaft stationary and the speed increase is obtained by the planet gears driving output shaft through the ring gear. In underdrive position, both direct and overdrive clutches are disengaged and power flow is through the countershaft which drives the planet gear carrier. The overruning clutch locks sun gear shaft and output shaft together causing them to rotate as a unit. This was for the 3 speed Hydraul shift. The Hydra-Power unit in direct the multiple disc clutch is engaged and a straight through power flow is obtained. In under the multiple disc clutch is disengaged and power flow is directed to the countershaft which reduces the speed of the drive unit output shaft. approximately 26 percent. Both units can be shifted on the go but I push the clutch in when I shift and also put the unit in under for gear selection in the manual transmission. I tried to shorten it up a little but you get the idea anyway.

Hi greg oliver,

Thanks!! That is exactly the type of description I was seeking.

Reason I'm curious is that I'm kicking around an idea to build an under drive for my 500. It would really handy for mowing when I get into heavy patches.

I sure wish Fiat had put something like that in the 1365. There's just way too much speed difference between low and high reverse in those tractors. The one thing that keeps them right off the edge of being the perfect loader tractor.