In an automatic transmission, the park position mechanically locks the vehicle's drivetrain to prevent the driven wheels from rotating.
The Core Mechanism: The Parking Pawl
The primary way an automatic transmission holds a vehicle stationary in park is through a component called a parking pawl.
Here's how it functions:
- Inside the transmission, there is a rotating shaft connected to the wheels (the output shaft).
- This output shaft has a notched wheel attached to it.
- When you shift the gear selector into 'Park', a small metal pin, known as the pawl, is engaged.
- The parking pawl locks the transmission's output shaft to the transmission casing by engaging a pawl (a pin) that engages in a notched wheel on the shaft, stopping it (and thus the driven wheels) from rotating.
Essentially, the pawl acts like a bolt sliding into one of the notches on the rotating wheel, preventing it from spinning and thereby locking the transmission and the wheels.
How it Holds the Car
Imagine a bicycle wheel with a piece of metal pushed into the spokes – it can't turn. The parking pawl works on a similar principle, but instead of spokes, it uses specific notches designed for this purpose.
- When the pawl is engaged in a notch, the transmission's output shaft cannot turn.
- Since the output shaft is connected to the drive wheels, the wheels are prevented from turning.
- This holds the car in place, even on an incline.
Practical Considerations
While the parking pawl is effective, it's a single point of failure and is subjected to significant stress, especially if the vehicle is parked on a steep hill or if another vehicle bumps it.
For this reason, it is always recommended to:
- Apply the vehicle's parking brake (also known as the emergency brake or handbrake) before shifting into park, especially on inclines.
- This allows the parking brake to take the majority of the load, reducing stress on the transmission's parking pawl.
- On level ground, shifting to Neutral, applying the parking brake, and then shifting to Park is a good practice.
By engaging the parking brake, you use a separate braking system (usually on the rear wheels) to hold the car, providing a backup and reducing the potential for wear or damage to the transmission's internal locking mechanism.
