How do speedometer gears work

The wires coming out of the bulb are turned at the end in opposite directions which ensures proper grip with the terminals of the holder. These wires make contact with the copper terminals as shown below.

The bulb is placed in the holder and makes contact with terminal 1 and 3 always. Terminals 2 and 4 provide the grip that keeps the bulb in position. This is just the external design; heart of the speedometer is inside the metallic cup where the speedometer assembly is placed. Knowing the parts will help in understanding the speedometer. After removing the screws, speedometer assembly and colored reflectors are visible. Four circular holes are cut on the metallic cup, smaller three for the bulbs and slightly larger one for driver cable holder.

This design allows the person to easily replace the defected bulb without disturbing other assembly. The colored reflector is responsible for converting the incandescent light into different colors on indicators. From the above parts we know basic designing of speedometer now, we will understand how rotation of wheel affects the speedometer? How is wheel and speedometer connected to each other.

The most important part that initiates the working of speedometer is a driver cable. The driver cable is like an input to the speedometer.

One end of the cable is connected to the wheel while other is attached to the bottom part of speedometer assembly. It rotates with the wheel and transfers this rotation to the speedometer. The image below shows the driver cable. When we cut the cylindrical bottom part vertically, we observe a pipe like structure with spirals at top as shown in the image below.

This is named as spiral groove. One end of Driver cable is positioned inside the spiral groove while the other end of driver cable is connected to the wheel.

The outer cylindrical part and the spiral groove are constructed in a way that allows them to move freely. Exact same cuts are made on both the parts. Role of driver cable ends with transmission of rotations to the spiral groove. The driver cable is inserted at the bottom part of the groove and is attached to gearing mechanism of the wheel. The rotation of wheel causes driver cable to rotate. The cable in turn rotates the spiral groove. Now, we move ahead to understand how the movement of needle takes place.

This is yet another part which proves that speedometer is a well thought design. How they have used the eddy current losses in working of speedometer. Entire speedometer assembly is held by a frame attached to the bottom part carrying spiral groove. There are two separate mechanism undertaking in device one to indicate speed and other to show distance.

Both the mechanisms are initiated by the rotation of spiral groove. As we know working of speedometer depends on eddy current, we will revise the concept on eddy current in brief. Speedometer mechanism assembly includes two cups, magnet, and pin with a spring around it.

The first cup shown in the above figure holds the magnet. The spiral groove passes through the first cup carrying the magnet. So, when the vehicle is in motion, its turns the speedometer cable which in turn rotates the spiral groove. The cup holding the magnet starts rotating with the rotation of spiral groove and generates a rotating magnetic field. We are unable to see the magnet in the cup as the magnet is covered from top by this second cup which is made of aluminum.

This cup is known as speed cup as the speed of the needle depends on this cup. As the ribbon moves, a mark on it lines up with the calibrated scale to give a reading. The other type of indicator using a straight scale rather than a dial has a barrel marked with a line.

The magnet causes the barrel to rotate until stopped by the hair spring. Again, as the barrel moves, the mark on it lines up with the scale to indicate the speed. On both these types of speedometer the ribbon or barrel is usually a different colour on each side of the marker line to make the speed easier for the driver to read.

In common with the trend towards electronic dashboard instruments, electronic speedometers are now becoming much more popular, even though mechanical ones are generally reliable. The most common type has a magnet attached to the gearbox output shaft and an electronic unit positioned close by to act as a pickup. Every time the rotating magnet passes the pick-up unit, the unit sends a pulse of electric current down a wire to the speedometer.

An electronic 'black box' inside the speedo uses these impulses to calculate the speed of the car. The Video Course teaches you everything about modern cars.

Inside a mechanical speedometer. Engine Block 18 minutes. Every part explained There's ridiculous detail on every part. Clearly and easily explained. To learn more, view our Cookie Policy. Contact Us. The following formula will work for any GM, Ford or Chrysler vehicle and is very useful when there is no established baseline from which to work from such as replacing a transmission in a vehicle that was purchased without a drive line.

The accompanying charts show what gears were manufactured for each type transmission. Note : Drive gear refers to the gear that is on the transmission output shaft. The driven gear is located in a removable housing usually in the tailhousing of the transmission.

Be aware that some transmissions will have different driven gear housings depending on the tooth count of the driven gear. Use the following formula to calculate the Tire Rev. Try this handy custom calculator. It will do all the work for you. If you don't know your tire diameter, calculate it below using the P-metric sidewall designation.

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