Why Are Gear Reduction Starters Replacing Direct Drive Starters?
Monday - 19/02/2018 16:50
When a vehicle’s starter motor begins to fail, a heavy strain is placed on your vehicle’s battery and other components in the starter circuit.
Repairing or replacing your starter motor at the first signs of failure saves you the added cost of replacing supporting components.
Direct Drive Versus Gear Reduction Starter Design
Until several years ago, most starters for trucks, buses and heavy vehicles were of the direct drive design, and were replaced with the same class of starter. Nowadays, you often have a choice since existing direct drive starters usually have a gear reduction replacement. There are advantages and disadvantages to each design. Understanding the differences ensures you will choose the correct starter for your vehicle.
Direct drive starter motors have a straightforward design. The motor armature and the Bendix drive, which engages the flywheel’s ring gear, are directly connected and positioned in-line with one another. There may be construction differences such as the use of field coils or permanent magnets within the motor.
Gear reduction starters use gears to reduce the speed of a higher-speed electric motor in order to extract more torque at the flywheel end of the starter. Two typical designs are Offset Gear Reduction and Planetary Gear Reduction. In the former, the motor armature and Bendix drive are not in-line, whereas in the latter they are lined up.
Pros and Cons of Each Design
Power – Direct drive starters require up to 50 percent more electrical power to turn over the car’s engine than do gear reduction starters. This means direct drive requires more current from the car’s battery, larger battery cables are needed and other components are designed to handle higher current.
Weight – Gear reduction starters are smaller and lighter than direct drive models. When replacing a direct drive starter with a gear reduction model, there is more space around it, which makes installation easier and may improve heat-resistance of the solenoid.
Maintenance – Gear reduction starters have a higher part count, but internal wear is often less compared to direct drive starters since they use bearings versus the bushings common in direct drive starters. Misaligned or worn bushings are the most common cause of slow cranking of hot engines that use direct drive starters.
Cost – Direct drive starters cost about 20 percent less than a comparable gear reduction model. However, "soft-start" gear reduction starters may require the additional cost of a magnetic switch installed in the solenoid circuit. The switch is required due to high current draw when the pinion gear initially engages the ring gear.
Speed – In general, gear reduction starters turn more slowly at the flywheel end than direct drive models. However, because their torque is higher, they have a faster speed when pistons are at TDC on the compression stroke where it is needed most. This is why they are ideal for diesel or other high-compression engines.