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Insight - How Does an Odometer Work

Written By: 

Bijal Parikh


An odometer measures and displays the distance travelled by a vehicle by sensing the rotations of a wheel. Odometer consists of a gear assembly and bunch of rings. A cable called the driver cable is connected from the wheel to the gear assembly.  When the wheel rotates the driver cable also rotates with the wheels that transfer the motion of wheel to the gear assembly which sets the gears in connected motion.


The gear assembly is made up of four gears in contact with each other. The image clearly shows position of each gear. On removing the gears we can clearly see the odometer as shown below.


The first ring of odometer is in contact with gear 4 which sets it in motion. To understand how the digits are moved in odometer assembly we need to have a clear picture of their design.

Let’s separate these rings and see how each ring is connected to other.


The odometer assembly is made up of six rings. Every ring is connected with the adjacent ring using the same arrangements. To avoid any confusion let us understand the mechanism between the first two rings.  The image shows the exact arrangement as to how the rings are interconnected. If we look carefully a white structure is visible. It is a small gear which is covered with the plastic plate. The plastic plate has a small cut through which we can see some of the teeth’s of the white gear coming out.


The image above shows the position and placement of the white gear and the plastic plate.



A small white gear is present inside the second ring and covered with a plastic plate. A small semicircular cut is made on the plate which allows the white gear in the ring to make conact with the white ring.  The shaft on the plate passes though the gear which allows the smooth movement of gear inside the ring.
Image of Odemetr

When viewed from the opposite angle the image shows the internal structure of the white ring. We can see the two teeth are carved out in the structure.


On removing the white ring we can see a clear picture of it from both sides. These are front and the back view of the ring. Every ring has a similar structure. One side has teeth running along the entire circumference while other side has only two teeth. 

The next image shows the entire arrangement of rings clearly.


Now, let’s understand how the two rings rotates in odometer assembly.

The driver cable sets the gear assembly into motion. The blue gear of the assembly is connected to the white ring as shown in the first image. This in turn sets the white ring into motion. As the white ring rotates, the two teeth starts moving on the outer circular track engraved on the black ring which allows smooth motion of the white ring.


After white ring completes 1 round (0-9) , the teeth on the ring will come in contact with the gear present in black ring and pushes the teeth of small white gear. Since the white gear and the black ring are connected with each other, the push on the gear sets the black ring in motion. An interesting question that comes to the mind is how come the black ring moves only by one point. This concept will be clear with the explanation of next part.

After completing one round the teeth on the white ring will come in contact with the white gear placed inside the black ring and in turn applies a force on it. Now we have seen earlier how the black ring and the white gear inside it are connected. This means that when the white gear will rotate, it will in turn rotate the black ring by one point. After black ring completes one round, ring 1 is released and similar motion takes place between ring 2 and 3. The process goes on till ring 6.

There is a very interesting fact about the design of white gear.


We have colored the teeth of the small white gear as black and red. It can clearly be seen from the image that the  black  colored tooth is  slightly more deeper  than the alternate red  one. The alternate tooth on the gear are specifically carved with different depths. Let’s understand why? After white ring completes one round, the teeth of the white ring comes in contact with gear.

The first tooth of the white ring pushes the black tooth of the white gear while red tooth passes by without making any contact with the red tooth of white gear. .This ensures that the movement of black ring only by one point. The second tooth of the ring is used as a precautionary measure in case the first one is damaged due to wear and tear. Odometer assembly works even when one tooth is damaged because the height of the teeth on small white gear is varied such that the second tooth of the ring passes without coming in contact.




nicE  job ......//