Is gear oil the same as hydraulic fluid

The most obvious answer to this question is that gear oil is generally intended for use in manual gear boxes and transmission fluid is for automatic transmissions. However, as is always the case with automotive lubricants, there are exceptions to this rule. In order to fully understand what the differences are between the fluids, we first have to take a look at the key differences between gearboxes and transmissions.

Both a gearbox and a transmission essentially do the same job in that they allow change of gears to control the speed, force and direction of travel, but they achieve this in different ways and so require different support fluids in order to operate properly.

Manual Gearbox

Manual gears operate based on a system of two shafts with gears which mesh together after user input via the clutch and the gear stick. This means that the manual gearbox creates a lot of heat, a lot of force and a lot of friction as the two moving objects come together.

As such, gear oil must be able to make these gear transitions as smooth as possible to prevent damage to the components as they shift and to achieve this gear oil has to have the following properties:

High Viscosity

The main thing you'll notice about gear oil is its viscosity. It's much thicker than motor oil, with an average motor oil being 5W/30 and the average gear oil being 75/90 (see this article for in depth explanation of oil viscosity index).

The high viscosity ensures that the entire gear train is lubricated thoroughly and, most importantly, that the gears are well cushioned from shock damage when they come together.

Resistance to Heat

The working of a manual gearbox creates a lot of friction and therefore a lot of heat. Gear oil is able to withstand high temperatures, transferring heat away from the gear train whilst not boiling off too rapidly and not depleting too fast to be of use for long.

Able to Function Under Extreme Pressure

Gear oils often come with extreme pressure additives in order for them to withstand the high pressures generated during the running of the vehicle, particularly where hypoid gears are involved. They help to keep the oil stable and functioning consistently.

Automatic Transmission

Automatic transmissions operate on in a planetary system where gears switch automatically depending on the demands of the engine. The gears in an automatic tend to be smaller and there are many more moving parts than a manual gearbox.

As such, the lubrication required is on a different level to that of a manual gearbox. Not only does the transmission fluid need to provide good lubrication but it also needs to be able to transfer power from the oil pump to the clutches which control the movement of the gears. To achieve this transmission fluid must have the following properties:

Low Viscosity

Transmission Fluid is essentially thin hydraulic oil. In order to effectively lubricate the delicate parts of the system, the viscosity of the oil is kept low - generally around 0W/5 or 5W/10. Most importantly it needs to be relatively free flowing in order to transmit power from the engine to the transmission. The challenge presented to transmission fluid to maintain lubrication, whilst keeping clutch engagement consistent.

Keep the Channels Between Components Clean

Detergent is added to transmission fluid to make sure that build-up in the channels is kept under control.

Resistance to Heat

Transmission Fluid acts as a coolant in a similar way to gear oil in that it transfers heat away from the mechanisms caused by friction and high pressure, however its boiling point is lower than gear oil and as a result requires additives in order to improve its life expectancy.

Anti-foaming

It is important that air is kept out of the fluid as this would interfere with the transference of force through the liquid to the transmission. As such, transmission fluid has anti-foaming properties to help combat this.

What does this mean for my vehicle?

Whilst some manual gearboxes do use transmission fluids in place of gear oil, the same cannot be said for putting gear oil into an automatic transmission. Gear oil is much too viscous for these systems and will cause clogging between components.

The golden rule when it comes to automotive fluids is to always use what is recommended by your manufacturer in your vehicle. Check your Owner's Manual for details on what is safe to put in your system if you are going to conduct maintenance work on your vehicle.

Hydraulic oil is different than other lubes. Not only is it a lubricant, it’s also the means by which power is transferred throughout the hydraulic system. So, it’s a lube and a power transfer device. This dual role makes it unique.

To be an effective and reliable lubricant, hydraulic oil must possess properties similar to most other lubes. These include: foaming resistance and air release; thermal, oxidation and hydrolytic stability; anti-wear performance; filterability; demulsibility; rust and corrosion inhibition; and viscosity in respect of its influence on film thickness.

To be most efficient in its role as a power transfer device, hydraulic oil needs high bulk modulus (high resistance to reduction in volume under pressure) and high viscosity index (low rate of change in viscosity with temperature).

As an analogy, consider the tension on a V-belt. If it is out of adjustment, the belt will slip. The result is a higher percentage of input power wasted to heat. This means less power is available at the output to do useful work. In other words, the drive becomes less efficient.

A similar situation can occur with hydraulic oil. Change in its bulk modulus and/or viscosity can affect the efficiency with which power is transferred in the hydraulic system.

As I have explained in previous columns, the perfect hydraulic fluid for transmission of power would be infinitely stiff (incompressible) and exhibit a constant viscosity of around 25 centistokes regardless of its temperature. Such a fluid does not exist.

Bulk modulus is an inherent property of the base oil and can’t be improved with additives. But viscosity index (VI) can be improved by using high VI basestocks such as synthetics and/or by adding polymers called Viscosity Index Improvers to the formulation.

Viscosity Index Improvers were first used to make multi-grade engine oils in the 1940s. These days, this common and well-tested technology is used to make high VI oils for other applications, including automotive transmission fluids and manual transmission gear oils. However, the VI improvers used in oils for the aforementioned applications are not typically shear stable when used in modern hydraulic systems.

But, recent advances in VI improver technology mean that mineral hydraulic oils with a shear-stable viscosity index in the 150 to 200 range are now commercially available.

While this may be good to know, what does it really mean to a hydraulic equipment owner? Well, within the allowable extremes of viscosity required to maintain adequate lubricating film thickness for hydraulic components, there’s a narrower viscosity range where power losses are minimized and, therefore, power transfer is maximized.

By maintaining the oil’s viscosity in this optimum range, machine cycle times are faster (productivity is increased) and power consumption (diesel or electricity) is reduced.

So, using a higher VI oil means the hydraulic system will remain in its power transmission “sweet spot” across a wider operating temperature range. You could think of this as similar to installing an automatic tensioner on the V-belt drive we talked about earlier in order to maintain optimum power transfer conditions.

However, based on simple cost/benefit analysis, if the cost to install the auto-tensioner was $200, we wouldn’t spend this money unless we were satisfied we can recover this investment – plus an acceptable return – through savings attributable to more efficient power transfer and/or reduced maintenance costs.

The same approach should be applied when evaluating the cost and benefits of using a higher VI hydraulic oil. But unlike the relatively simple V-belt drive, savings accruing from increased hydraulic machine performance can be more difficult to quantify.

The results, though, of field trials conducted by a manufacturer of shear-stable VI improvers1 have demonstrated real economic benefit to the equipment end-user. In one trial, the performance of a 40-horsepower compact excavator was evaluated using an all-season 142 VI “baseline” oil and compared to the performance of the same machine using a 200 VI “test” oil.

The test procedure was as follows:

1. Run baseline data with 142 VI oil.

2. Start with a new air filter and fuel filter.

3. Top off fuel to fill the neck at start of test.

4. Put trenching blade width to normal depth.

5. Dig trench for seven hours.

6. After seven hours, record fuel to refill.

7. Measure trench width, depth and length.

8. Repeat steps 2-6 with a second operator.

9. After baseline is established, change oil and filter, run for two hours, and repeat oil and filter change with 200 VI oil (due to some dilution of the 200 VI oil with the 142 VI baseline oil after changeover, the actual VI of the “test” oil was less than 200).

10. Repeat steps 2 through 7.

The higher VI test oil demonstrated the following advantages over the baseline fluid:

• 5.4 percent improvement in “fuel economy” – cubic yards of dirt moved per gallon of fuel consumed.

• 4.3 percent improvement in “productivity” – cubic yards of dirt moved per hour.

To assign a value to these performance gains, a spreadsheet was developed to calculate an owner’s variable costs over the 1,000-hour drain interval recommended by the excavator OEM. The following assumptions were made:

• All-season baseline oil cost $9 per gallon and the 200 VI test oil cost $18 per gallon.

• Labor and equipment rental cost was $75 per hour.

• Diesel cost was $3.15 per gallon.

From extrapolating the results of the trial, it was determined that with the baseline oil, the excavator could dig approximately 20,000 yards of trench in 1,000 hours. And, the same amount of trench could be dug in 874 hours with the 200 VI test oil. No value was assigned to the additional 126 hours the machine owner would have to undertake additional work.

Based on the field test results and the assumptions stated previously, replacing the 142 VI all-season oil with 200 VI oil would save the machine owner $10,000 for every 1,000-hour drain interval (see Figure 1).

As the figure shows, while the fuel cost savings are not insignificant, the greatest potential benefit from switching to higher VI oil is likely to accrue from machine productivity improvement.

Read more about hydraulics best practices:

The Seven Most Common Hydraulic Equipment Mistakes

How Do You Know if You're Using the Right Hydraulic Oil?

The Benefits of Maximum Efficiency Hydraulic Fluids

Symptoms of Common Hydraulic Problems and Their Root Causes

Reference

1. Gregg, D., Herzog, S.N., “Improving Fuel Economy and Productivity of Mobile Equipment through Hydraulic Fluid Selection: A Case Study” NCFP 08 – 2.4, IFPE March 2008, Las Vegas, NV, USA

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