Prime Mover Magazine


How low can you go?

How low can you go?

The addition of super-low crawler ratios to the Volvo I-Shift transmission has drastically widened the abilities of Volvo trucks in the heavy haulage space as well as adding some advantages for operators dealing with more conventional loads.

The concept of crawler gears is not a new one. Since the inception of commercial vehicles early in the twentieth century the need to move high weight loads with the limited horsepower available at the time resulted in the adoption of transmissions that reduced the wheel speed in comparison with the engine’s crankshaft speed thereby allowing the engine to perform the task, albeit often at a snail’s pace.

Planetary reduction diff hubs, auxiliary ‘joey’ gearboxes and deep reduction options on multi-speed transmissions contributed to the ability of trucks to shift heavy loads, often with a significant trade off in road speed, fuel economy and adding a driving challenge that was often well beyond the skills of many drivers which then brought with it safety concerns.

Horsepower is no longer much of an issue as the 16-litre Volvo engine demonstrates with it being possible to specify it up to 700hp in Australia.

Volvo’s I-Shift 12-speed automated manual transmission is by now a mature piece of technology that dominates the Swedish manufacturer’s driveline space.

According to Ove Wikstrőm, the Volvo engineer known as the “Father of the I-Shift”, during 2018 the automated transmission accounts for 97 per cent of global sales of the Volvo FH and FM models. Even the North American market is running at over 90 per cent and Ove seems convinced that Volvo will be discontinuing manual transmissions in the Heavy Duty (Class 8) category very soon.

To take the transmission a step or two further, Volvo has developed an adaptation that incorporates an additional section between the main gearbox and the input shaft and bellhousing to provide the extra low transmission ratios that are integral to the crawler gear engineering.

The main I-Shift transmission has been reinforced as well with the addition of the crawler gear option.

The extra hardware adds 46kgs to the weight of the truck and extends the overall length of the transmission by 120mm.

The I-Shift is a relatively short transmission anyway so the length doesn’t pose any problems. The standard tail shaft rated at 28,000Nm is replaced with a 33,000Nm rated shaft which adds a further 18-27kgs depending upon its length which is determined by the truck’s wheelbase.

The crawler gear option is currently available with 13-litre and 16-litre engines. There are several variants of the crawler gear set up including an ultra-low crawler package with multiple speed reverse gears.

The lowest forward ratio is 32.04:1, which is possible with both direct and overdrive I-Shift transmissions.

Our first driving experience with the crawler gears is sensibly taking place on a closed circuit using two FH16 prime movers both rated at 700hp and connected to multi-axle low loaders with one loaded to around 130 tonnes and the heaviest grossing 203 tonnes.

Driving with these weights requires some adjustments to driving style. The advice is to only engage the crawlers when necessary and to select manual mode.

Under normal highway conditions with typical gross weights around 60 tonnes an experienced Volvo I-Shift driver will travel 90 per cent of the time in auto mode and only 10 per cent in manual when negotiating roundabouts and crests.

In the heavy haulage space and with crawler gears, the inverse now applies with manual control preferential to avoid a gear change in an inappropriate situation.

The traction control also is switched off because a loss of traction can apply the brakes on different hubs which will reduce torque which is the last thing we want on a grade.

At this level of weight we also engage the power divider to make use of the inter axle differential at all times.

Starting off on the flat the Volvo’s weight and inclination sensors are able to automatically select the forward starter gear but manual selection is required for the selection of up to six ultra-low crawler speeds in reverse.

At more than 200 tonnes gross weight the truck accelerates steadily as we initiate gear upshifts at 1,100rpm. Stretching out to 1,500rpm we can actually skip a gear by flicking the lever twice.

The rear axles on the test trucks are fitted with 4.12:1 gears in addition to the hub reduction gears. Up to 7.2: rear axle ratios are available in cases of consistent high loads where overall road speed isn’t an issue.

The driveline specs of the test trucks permit startability at 190 tonnes gross vehicle weight on a 10 per cent inclination. We get to try out 203 tonnes on one of the steepest rises on the Mt Cotton course which is around nine per cent.

At this sort of weight few would plan a stop on such an incline, but the reality is that driving on public roads often throws up obstacles to forward progress such as unexpected traffic or non-advised road works.

At this weight and grade the Volvo’s hill start aid could possibly hold on a bit too long and actually hold the truck back, so the experts’ advice is to use the trailer brake handpiece or even better, the park brake by holding the dash mounted lever out 5mm and accelerating steadily and releasing the brakes as the truck makes the slightest move forward.

Once moving we use the engine control, which is Volvo’s version of a hand throttle, to set the road speed. The engine control function adjusts the engine revs whereas the cruise control is road speed sensitive.

The engine control buttons on the steering wheel include a toggle for fine adjustments and can be handy at speeds of less than 20 km/h.

Moving off in the lowest of the crawler gears we can tick along on idle at 700rpm at a speed of around 900 metres per hour.

If we were riding a push bike we’d fall over.

The pyrometer gauge shows no heat coming from the turbo, there is no heat in clutch and the gearbox oil temperature remains steady at 71 degrees.

Volvo claim that even at 190 tonnes on a 10 per cent grade there is 75 per cent less energy going through the clutch, which is an indication of how under-stressed the driveline is when the ultra-crawler ratios are engaged.

After the haul to the top of the rise we select the full diff cross locks and begin negotiating the descent.

The purpose of this is to spread the retardation force of the engine brake across all drive wheels which will reduce the chance of wheel chatter or even compression lock-up if all of that energy is directed at just one wheel.

In situations where it can be legally done, experienced heavy haulage operators will often also lift the first two or three axles on the float to transfer more weight to the drive wheels.

The slowest possible forward speed with a conventional I-Shift is 1.9 km/h in forward and 1.7km/h in reverse whereas the ultra-low crawlers permit 0.9 km/h in forward and an even slower 0.8km/h in reverse.

The obvious applications for the crawler gear transmission include high gross weight combinations, road trains and mining trucks.

The low reverse ratios are especially handy when connecting trailers which expands the transmission’s appeal to more general freight work.

The specification of crawler gears can dramatically increase the flexibility of a prime mover and make it suitable not just for heavy haulage demands, but it can handle the point-to-point and fuel economy tasks required for traditional B-double applications.

The basic premise is to specify the crawler gears for startability and reduced driveline component stresses and utilise diff ratios suitable for maintaining highway speeds and good fuel economy when grossing less than 70 tonnes.

It genuinely presents a ‘have the cake and eat it too’ advantage.

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