Functioning of transmission

[Uncia_Uncia]

I was dealing with one of engine manufacturer and one thing led to another. The manufacturer asked me about bell housing requirement of the engine, which led me to the transmission of the agricultural forklifts. It was not much hard to figure out from transmission manufacturer's site about bell housing requirement. ( JCB - SS640)
Out of curiosity I started getting much into transmission. Couple of things I don't understand about transmission are :
1) How different is Power shift transmission (JCB-PS750) vs Synchromesh Powershuttle transmission(JCB- SS640)? Not just in cabin for
operator, but how its differently built and function?
2) Why transmission manufacturers keep adding more and more speed into gear. I know that driving in last gear is always fuel efficient,
but how its helpful adding more speeds. 4 speed forward vs 6 speed forward, How different it would be in performance?

 

[MikeHalloran]

1) Get the appropriate shop manuals and study them, and watch a teardown if you can.
Synchromesh in general implies presence of a 'balk ring' in the dog clutches of a transmission, intended to drag the rotating bits to a common engagement speed during a shift, but the other marketing phrases may be specific to JCB.

2) More gears allow better matching of the engine's torque curve to the load demand.

My 2001 Camaro gets ~31 mpg at 55 mph on level ground in sixth gear (1:2 OD), but it doesn't like climbing hills, accelerating, or cruising slower in that gear. In traffic, I don't use sixth below 80 mph.


Mike Halloran
Pembroke Pines, FL, USA

 

[romke]

in agricultural and earthmoving equipment powershift transmissions are extensively used because it allows changing from one gear to another with only a very short break in power throughput. that is important if the vehicle is used in rough terrain - if changing gear takes too long the vehicle would come to a standstill immediately...full powershift transmissions however are costly and therefore in some transmissions there is only one powershift step, allowing to change gear immediately between high/low.

the reason for more gears is to compensate for the shortcomings of the internal combustion engine. especially in off road use you want to have all the power available at any vehicle speed - whereas the engine can only deliver maximum power at a very high rpm, thus you need to have more gears to be able to match the power requirement to the vehicle speed. take also into account that for example a farm tractor will be used at very different speeds depending on the actual operating activity: ploughing, tillage and transport activities are done with quite different speeds - but may need nonetheless about the same amount of power, and hence the many gears needed to match the engine output to actual requirements.

 

[Uncia_Uncia]

Thank you very much for replying back.
The answers for second question is pretty much understood but to be honest still confuse about the difference between Power shift Transmission vs Synchromesh Transmission. The one thing I did notice is that in the technical configuration of Transmission, power shuttle is mentioned next to speed "4 Speeds forward / 4 Speeds reverse with Powershuttle".
Even in the case I simply understand power shift vs synchromesh, there is some missing link.

1)

http://www.jcbdrivetrainsystems.com...s/Synchromesh/SS620-2WD-SERIES/SS640-4WD.aspx

2)

http://www.jcbdrivetrainsystems.com/Products/Transmissions/Power-Shift/PS750-SERIES/PS750-4WD.aspx

 

[BrianPetersen]

Synchromesh is like a normal automotive manual transmission. In order to change gear, the power flow through the transmission has to be interrupted momentarily. Clutch disengage, move one synchro unit out of engagement and move the next into engagement, clutch engage. The fact that there appears to be a torque converter involved in this particular transmission doesn't change this - one way or the other, it has to completely unload torque throughput somehow, in order to disengage the synchro unit on one gear and engage the synchro unit for the next one.

I don't know the details of the power-shift transmission that you're showing, but it is pretty likely to be a variation of an automotive torque-converter automatic transmission in which hydraulically operated multi-plate friction clutches are used to select each gear and it is possible for them to momentarily "slip" during engagement or disengagement, thus allowing (almost) uninterrupted torque transmission, OR it is a variation of an automotive dual-clutch transmission which uses synchro units to engage each gear but in combination with TWO friction clutches that are capable of momentary slip - and alternating forward gears are connected to each friction clutch (1, 3, 5 on clutch A and 2, 4, 6, R on clutch B or something like that) and a gearchange occurs by pre-engaging the synchro unit on the next gear (no torque transmitted during this process) and then disengaging (let's say) friction clutch A simultaneous with engaging friction clutch B.

 

[romke]

Powershuttle in this case most likely applies to a additional unit that makes it possible to reverse gear with a very short shift time - thus when moving forward in a particular gear, you immediately can move backward using the same gear ratio. Given the use of the vehicle involved that can come in very handy, speeding up operations.

 

[gearcutter]

Powershift transmissions do not have mechanical linkages (yokes, rails, detents, etc) to perform the change-of-gear function. They are constant mesh gearboxes that have individual clutch packs for each gear. The clutch packs are hydraulically activated. There is no synchromesh.

Earlier machines used synchromesh gearboxes but had 'shuttle reversers' between the torque converter and gearbox. Also electro-hydraulically activated and utilising planetary gears and clutch packs. It allowed fwd/neutral/rev to be selected by a lever under the steering wheel. A very handy device for machines like front-end loaders, backhoes, etc that required frequent changes in direction.