In the last blog about differentials, we discussed how important it was to regularly service your diff and the reasons behind that. Now and in further blog posts we’ll go a little further and look at some of the common types of diffs, how they work and how their differences can have an impact on the gear oil you should be using.
First of all, let’s go through the basic operation of differentials. A diff in a vehicle whether FWD (Front Wheel Drive), RWD (Rear Wheel Drive) or AWD/4WD (All Wheel Drive/Four Wheel Drive) is almost always designed to do exactly as the name suggests. It must differentiate torque and rotational speed between the passenger side and driver’s side wheel on that axis when the vehicle is cornering but allow the same torque and speed while the vehicle is travelling in a straight line. Now you may be wondering why this is required. Think about when you turn a sharp corner in your car, the wheel on the outside has to travel further than the inside wheel, but it must do so over the same amount of time, so how is this possible? The outside wheel must travel at a higher RPM than the inside wheel!
Obviously, this couldn’t be done if the inside and outside wheel were fixed on a single axle, so a differential gear set is required. An open (conventional) diff is designed to do exactly this and is one of the easiest to get your head around. The major components of this type of differential in an RWD vehicle are the pinion & ring gear set, the carrier, the cross pin, the spider pinion & side gear set and of course the bearings and axles to drive each of the rear wheels.
The tail shaft from the gearbox drives the primary gear reduction in the diff which is the pinion & ring gear set. This gear reduction is what gives you the significant speed reduction and torque increase ratio of the diff (Eg: 3.6:1, 3.9:1, etc.) and also transmits torque and rotational speed about a 90° angle from your tail shaft to the rear axles. The pinion & ring gear set are typically bevelled gears forming the 90° angle that have spiral cut teeth, usually called ‘spiral bevel’. In this case, however, the axis of the pinion & ring gear is also offset by further tooth engagement and strength, so it is then given the name ‘Hypoid’, see image below.
Due to the spiral cut teeth and the offset axis of the Hypoid gear set, there will always be a significant sliding/scuffing action taking place between teeth surfaces when the gears are meshing which will cause lots of friction. This is one of the reasons why differential oils usually contain a certain level of EP (Extreme Pressure) additives which we discussed in the previous diff blog. The level of EP additives required depends on the diff design including the materials used, the torque transmitted, tooth profile & axis offset. The vehicle manufacturer will specify this in the vehicle handbook, usually as GL-3, GL-4, GL-5, etc.
Now, this is where it can start to get a little confusing, but we’ll do our best to explain…
The ring gear is connected directly to the carrier, usually by a series of bolts. The carrier houses the cross pin, spider pinions and side gears which do all of the tricky torque and speed splitting, see image below. The side gears are connected directly to the rear wheels typically via splined axles, so each will always rotate at the same rate as the wheel it is connected to.
The cross pin is fixed to the carrier axis at 90° so as the ring gear & carrier rotate; the cross pin flips end over end with them. The spider pinions are on bearings on the cross pin and are free to rotate independently about its axis. The end over end action of the cross pin causes the spider pinions to drive the side gears which then drive the rear wheels via the solid splined shafts. Because the spider pinions are free to rotate on their bearings about the cross pin axis, this allows the side gears to travel at different speeds to each other if required! Confused yet?
A simple way to think of it is when the vehicle is travelling in a straight line. The spider pinions will be driving both side gears at the same speed but not rotating on their bearings about the cross pin axis, just going end over end with the cross pin. During vehicle cornering, however, the spider pinions are then forced to rotate on their bearings as well as going end over end with the cross pin due to the difference in speed of each wheel. Due to this design, one wheel can even be completely stationary while the other is rotating. Ever seen an RWD car with an open diff do a burnout and only one wheel spins? This is why!
The biggest disadvantage of an open (conventional) diff is in wet conditions or where traction is an issue such as on sand or gravel surfaces. Open diffs can be particularly problematic in towing applications due to the open diff’s design allowing torque to be delivered by only one wheel. The open diff will always produce torque via the path of least resistance which is usually the wheel on the slipperiest surface. This, of course, is not ideal as the vehicle will only spin this single tyre without driving forwards or pulling the load.
So what about gear oil for open differentials? Open differentials are the simplest type of diff to choose gear oil for as they do not contain clutches like some LSDs do. Just select the correct viscosity grade (SAE 80W-90, SAE 80W-140, etc.) along with the proper performance specification (API GL-4, GL-5, etc.) gear oil of high quality such as the Nulon brand. In some cases such as when towing a heavy load (caravan, large boat, etc.) using a gear oil of a higher viscosity than usually specified can be beneficial in reducing gear wear within the diff. Another way to achieve this is through the use of an excellent anti-wear additive such as the Nulon G70 Manual Gearbox & Diff Treatment which if PTFE (Teflon) based.