It’s easy to take the axles of your backhoe loader for granted. When, for example, was the last time you crawled under your machine to inspect the axles? When is the last time you even thought about them?

Just because axles are the strongest part of your machine, that doesn’t mean you can ignore them without paying a price in costly downtime and repair bills. Axles operate in a harsh environment of damaging dirt, sand, mud and water. They are a sophisticated system of ring, pinion and planetary gears, differential locks and wet disk brakes. These components can be subject to high rates of wear if not maintained properly.

Diego Navarro, service marketing manager for John Deere Construction and Forestry, explains the whys and hows of giving your axles the attention they deserve.

A look inside

The axles of a backhoe loader or other construction machine perform several functions. In addition to carrying the weight of the machine and connecting power from the engine and transmission to the wheels, the axle has to absorb the shock loads generated from traveling over bumps and ruts and from using the bucket or other attachment. “When you drive the loader forward and the bucket hits a pile of dirt, the shock of hitting that obstacle passes through the axle,” Navarro notes.

The differential consists of a set of gears that transfer the rotational power of the driveshaft 90 degrees to the right and left axle shafts. These gears make it possible for the rear wheels to rotate at different speeds when turning the machine. The differential allows the wheel on the inside of a turn to rotate more slowly than the outside wheel, which must travel farther in the same time to complete the turn. Meanwhile, the differential provides torque to wheel with the least resistance.

Most backhoes are equipped with a differential that physically locks together the left and right axle shafts to prevent wheel slippage for more traction. Some, however, use a limited-slip differential, which permits one of the wheels to slip to a certain extent.

Because the two axles are not completely locked together, part of the torque also goes to the nonslipping wheel. The limited-slip differential offers the advantage of allowing the machine to change directions while transmitting power to both wheels. Also, unlike a locking differential, it works automatically, eliminating the need to engage or disengage it when operating the machine.

Four-wheel-drive backhoes typically have a locking differential in the rear axle and a limited-slip differential in the front axle.

Brake differences

Although some manufacturers use external dry disk brakes mounted at the end of the axles to stop wheel rotation, most equip their machines with internal wet disk brakes.

Unlike external brakes, which are exposed to dirt and water that can shorten brake life and reduce braking effectiveness, wet disk brakes are mounted inside the axle housing and immersed in oil, which dissipates heat. They may be placed close to the differential to accommodate machine geometry or farther away near the ends of the axle for better cooling.

Planetary gears, located after the differential in the drive train, reduce the speed of the axle to provide more torque for working. Depending on make and model, they may or may not share the same oil as the differential.

Backhoe jobs involving a lot of braking could lead to overheating of the gear oil or tractor fluids. Working in mud or other conditions that leave a thick coat of dried material around the axle can cause overheating.

“I’ve seen axles where you needed a screwdriver to break the crust around the axle,” Navarro says. “I’ve seen axle bearings fail because the oil got too hot, reducing the viscosity and lubricating ability of the oil.”

Maintenance tips

Now that you know a little more about how axles are built, here are some basic tips for maintaining them.

Use the correct lubricants. Depending on the manufacturer, either tractor fluids or gear oils are recommended for lubricating the gears and internal brakes. They also reduce brake chattering. “In general, gear oils are heavier than tractor fluids, which are also used in hydraulic transmissions,” Navarro says. “Either way, they need to be good quality because they are working in extreme load applications.”

He recommends using the proper viscosity for the seasons, such as 75W-90-, 80W90- or 85W140-weight oils or tractor fluids with a 32 or 68 ISO viscosity. If using gear oil in axles with wet disk brakes, the oil should include a friction modifier, he adds. A friction modifier usually isn’t necessary when using tractor fluids because they already include it in the formulation

Check the oil or fluid levels. Most differentials have a plug that can be removed with a socket wrench for checking lubricant levels. “If the oil or fluid is cold, you should be able to stick your finger in the hole and feel it,” Navarro says. “If the lubricant is warm, the level should be up to the hole. If it isn’t, you may have a leak.”

Analyze the oil. An oil analysis is the best way to check lubricating oil or fluid for viscosity and acid number. It can also reveal the presence of dirt, water and metal particles, which can indicate wear or failure of components. Navarro advises doing an analysis every 500 hours.

Change lubricants when needed. The gear oil or tractor fluid should be changed based on its condition, rather than hours of use. “If you change it based on hours, then you don’t account for any work done in high-stress environments, like working in water, which can shorten the effective life of the lubricant,” Navarro says.

Check the breather. Backhoes often have a breather located at or near the top of the axle. It allows the heated oil to expand, pushing air out along with any moisture in that air to reduce water contamination. Clear any debris from around the breather.

Tractor fluid or gear oil around the breather opening could indicate failure of the seal around the hydraulically operated piston that actuates a wet disk brake.

Keep wheel bolts tight. While not strictly an axle component, the wheels of your machine are directly connected to it. The wheel studs use friction to keep the wheels clamped to the axle flange. “When the wheel studs are loose, you lose that friction and all the load is transferred to them and not the axle,” Navarro says. “That’s when the studs start to break. The studs are designed to keep things together, not to support the weight of the machine.”

Greg Northcutt is a freelance writer based in Port Orchard, Wash. He can be reached by e-mailing this publication at editor@onsiteinstaller.com.