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How Do Semi Truck Axles Work? (17 Facts You Should Know)

The axles on a semi truck work by sharing the weight of the tractor and trailer.

There are a few different types of axles, all of which we will cover along with how much weight they can legally take.

What are steerable axles, and can you drive on a bent axle? Stick around… we have the answers.

Here’s a short summary as to how Semi Truck Axles Work:

Semi truck axles are part of the driveshaft and differential system that work by allowing the truck’s wheels to turn at different speeds when cornering. Steering axles steer the truck to improve the turn in tight situations. Additionally, axles aid in the overall weight distribution of the load.

What are Semi Truck Axles and How do they Work?

The axle is a central shaft that is connected to the wheels.

It is connected to the differential and driveshaft and can operate independently from one side to the other meaning. The wheels can turn at different speeds.

They work by supporting the weight of the vehicle’s load at the wheelbase.

It can be hydraulically lifted and then lowered when needed and can even be used as a steering aid.

How Many Axles do Semi Trucks have?

A semi truck is a large long vehicle that needs to distribute a lot of weight.

It has five axles in total. Two to three of those will be found on the trailer, depending on the truck model.

One of the trailer axles can be lifted when the load is lighter, and to reduce wear on the tires, brakes, and axle itself.

Every manufacturer has adopted this method to reduce greenhouse gases.

What Components are on a Semi Truck Axle?

There are many components to an axle. From the larger shafts to the smaller.

But just as important as the stub axle and seals. You will find a cross shaft, pinion gears, input yok, side gears profile, a helical drive gear.

The helical driven gear is connected to a pinion shaft, the bevel gear, and the differential case. You also have an output shaft and a spindle.

Which axle Drives on a Semi Truck?

The drive axle is the main one. It is connected to the engine from the driveshaft, gearbox, and differential. It allows the truck to move forwards and backward.

Without the drive axle, there would be no movement at all.

The drive axle can take weights of up to 4,250 pounds per tire. As a result, these tires are usually in full contact with the road’s surface.

They tend to be the first set of tires to wear out.

How many Drive Axles does a Semi have?

The average daily truck we see on our roads will typically have just one drive axle.

Two drive axles are found on trucks with more demanding jobs like mining or logging and oversized loads.

These trucks need the extra help to maneuver in sometimes smaller spaces. With rough terrain, two axles make it easier than one axle would.

What does 6×2 axle Configuration mean?

A 6×2 configuration refers to the number of axles a tractor has. In this case, it will have three axles.

One of these has the power, the other will be liftable, and the third will be fixed.

We see these configurations on our day-to-day trucks. Sometimes we see them with the rear/center tires up. 

What are Tandem Axles in a Semi Truck and How do they Work?

The tandem axles of a semi truck, are recognized by the two wheels on each side of the axle.

Although this isn’t always the way, this allows the trucks to haul more weight and provides the trucks with better traction.

The truck will have a smoother ride and far better stability. Both the axles will usually be drive axles to propel the vehicle and provide extra power to carry a heavier load.

What are Pusher Axles in a Semi Truck and How does it Work?

Pusher axles are commonly found on vehicles that require heavier loads and have shorter bodies, like a dump truck.

The pusher axle increases the load capacity of the truck.

It is positioned in front of the drive axles and provides added support to the chassis while the vehicle is turning.

How much weight can a Pusher Axle Carry?

The weight a pusher axle can carry at any one time is 20,000 pounds. The pusher axle takes some weight away from the rear axle and transfers it to the front axle. It allows weight to be moved from one section of the truck to another safely.

What is a Tag Axle on a Semi Truck and How does it Work?

The tag axle on semi trucks is under the rear of the truck’s body. It is in place to make it easier for the vehicle to move around with a shorter wheelbase.

The tag axle is usually raised when the truck doesn’t have a load. But must be lowered to give extra stability when it does.

If you forget to lower the axle, you should pull over and do this as soon as it is safe to do so.

The raising and lowering are controlled by hydraulics, using a switch located on the side of the cab, below the driver’s door.

How much weight can a Tag Axle Carry?

Tag axles can carry a maximum weight of approximately 40,000 pounds. If there is a shorter space between axles, then they can hold up to 34,000 pounds. These weights can vary from truck to truck, and the laws can change slightly from state to state.

Spreading weight over additional axles means the truck can carry more weight and have more control.

What are Drop Axles in a semi truck and What do They Do?

The drop axle is found in tri-axle and quad-axle trucks and is used to support any additional weight of the vehicle.

They should only be used when the truck’s other axles are at their maximum capacity.

It can be operated via the switches on the side of the cab below the driver’s door. Each switch has a pressure control valve to control the airflow to the axles.

The gauges to these valves are found on the floor of the cab by the driver’s seat.

How Much Weight can you put on a Drop Axle?

Each truck has a different axle weight limit, it may vary with the style of truck, and even the length of the vehicle will make a difference to the weight for each axle due to the spaces between each one.

The maximum weight you can put on a drop axle is around 34,000 pounds on the rear. And 16,000 to 22,000 pounds if the drop axle is in the center of the truck.

Remember, not every state has the same weight charts, and not every vehicle has the same specs. Check the regulations and specs before loading your truck.

What is a Steerable Axle and How does it Work?

The steerable axle provides the truck and trailer with additional maneuverability.

It works by steering one or more of the axles in the opposite direction to the front wheels to shorten the turn.

It allows the truck to take a tight turn easier. The steerable axle can be switched off by the flick of a switch to avoid using it unnecessarily.

How much weight can a Single Axle semi truck Pull?

The maximum weight a single axle can pull is 20,000 pounds. A tandem axle has a maximum weight limit of 34,000 pounds, and the gross vehicle weight limit is 80,000 pounds. The weight is carefully calculated to allow the vehicles to stop and corner safely, considering the load and emergencies.

How do I know my Axle Capacity in a semi truck?

There will be a VIN plate or sticker with the information you need.

These can be located inside the glove box, on the driverside front frame rail, above the sun visor, or the driver side door frame.

If you can not find them in these places for some reason, contact your manufacturer to find out what to do.

Can you Drive on a Bent Axle?

It is not advisable to drive on a bent axle.

The best thing to do is to get towed and have the axle repaired or replaced. Depending on how bad the bend is in the axle, it could cause further damage to other components.

The bend can occur in an accident, running over a curb, or hitting a pothole at high speeds.

The damage could leave you with more downtime and may not be under your warranty.

Happy Trucking!