Parts of a Semi Truck Diagram

Parts of a Semi Truck Diagram

Those involved in maintaining, servicing, and repairing semi trucks must understand the systems and components that power, slow, stop, control, direct, support, and stabilize the 18-wheeler.

Currently, the main primary source of power for semi trucks is the diesel engine. Some fleets have opted for natural gas sources such as LNG, but it does not have the same level of infrastructure built to accommodate fueling and repairs. Regardless of engine type, all engines built in today’s time are computer controlled.

Distance variations

There are specific names and terms given to distances between certain parts on a semi-truck. Common term and phrases include:

  • Wheelbase – The distance between the center of the steer and drive axles
  • Trailer-length – the overall semi-trailer length. This is usually 48FT or 53FT.
  • Overall wheelbase – The wheel base between the centerline of the steer axle and the rearmost axle of the trailer
  • Landing gear clearance – The landing gear clearance is the center point to the nearest point of interference.
  • Kingpin setting – the front of semi-trailer to the center of kingpin on semi-trailer.

Electrical Systems

The truck’s electrical systems are comprised of its batteries, alternator, and starter that must be in accordance with the engines requirements. Without the electrical systems certain features of the truck would not work including the starter, lighting, windshield wiper motors, computer system operation, and the cabin amenities such as a power inverter.


Majority of heavy trucks have a push or pull clutch. Some trucks have centrifugal or wet clutches. Trucks with higher torque ratings have a two-plate clutch. The purpose of a two-plate clutch is to increase the friction area and transmit it to the transmission without any slippage. Automated manual transmission equipped trucks have clutches that are managed electronically.

Torque Converters

Torque converters are found in fully-automatic transmissions such as the ones made by Alison. It is like a clutch for manual trucks but works in a different way. Torque converters are classified as fluid couplings because they use hydrodynamics.


Heavy-duty truck transmissions are manual, automated manual, or automatic. Various components of the transmission must provide superior performance for hundreds of thousands of miles. These components of the transmission include the gears, shafts, forks, and bearings.

Manual transmissions have two or three countershafts that transmit engine torque from the input to output shaft. The countershafts distribute torque two or three directions, and this results in reduced strain on individual gears.

Automated manual transmissions do not have a clutch pedal because the clutch is actuated automatically. The I-SHIFT transmission by Volvo is an example of an automated manual transmission.

Drive Shafts

The driveshaft is a tube with flanges or end yoke that is attached to the output shaft of the transmission and it helps with maneuverability of the drive axles. Parts of the drive shaft are interconnected with each other and with the transmission with universal joints which allow torque to transmit


Axles give the suspension, wheels, and steering parts a place to be mounted. The drive axles have differential gear shafts which help truckers in the snow prevent their truck from sliding. The front axle is called the steer axle. The rear-two axles are called the drive axles.

Steering Components

Two main types of steering systems are found in heavy trucks. They are manual and power steering assist systems. Manual steering systems have worm roller or recirculating ball gears. Power steering assist use a hydraulic pump. Rack and pinion steering are found in newer trucks that provide better control.

Air Brakes

Air brakes is the most common type of brake system on a heavy truck. Air brakes use a complicated network of pneumatic lines, valves and cylinder that maintain the transfer of compressed air to the brakes.

Chassis Frame

The chassis frame includes the rails and crossmembers of the truck and the strength of these components impacts the truck’s durability to carry heavy loads. Frame designs vary by manufacturer.

Fifth Wheel

Fifth wheel is the part of a semi-tractor that connects to the kingpin of the trailer. The type of connection formed between the coupling of the fifth wheel and kingpin allow the truck to articulate when turning. Fifth wheels can be positioned or slide backward and forward to allow proper distribution of the trailer weight on the drive axles of the tractor.

Heating, Ventilation, Air Conditioning (HVAC)

HVAC systems are responsible for climate control of the tractor unit. Some HVAC systems are coupled with auxiliary systems that help balance the in-cabin temperature with engine temperature and engine shutdown features to prevent over-heating. Anti-idling laws are creating an increased need for sophisticated HVAC systems. For example, the development of parking heaters has aided in preventing trucks from idling for extended periods of time.

One Reply to “Parts of a Semi Truck Diagram”

  1. My name is Derrick from Chicago. I need a book on diagram semi truck parts . I attended Truck Driving School. And I’m doing my pre-trip inspection. I would like to know do you have any books on semi truck diagrams parts.

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