Wednesday, July 22, 2009

Delco 38MT Starter Planetary Gear System

Planetary Gear Systems have been around for awhile. A very common use for them are automatic transmissions. The gear ratios can be altered by changing which gear is driving and which gear is being driven. There is always one of these 3 gears that has to be fixed.
This Delco 38 MT starter pictured has been operating in a 2007 C2 School Bus. You can see the rust forming on the terminals. This is common since starters are placed low in the engine and close to the elements. We generally remove the starters on all our units every 2 years and give them a check over.

We lubricate the bearings/gears and check brushes plus wiring.
On this model the solenoid and relay switch(fastened to the side) are non-serviceable so they have to be replaced if there is any doubt on their mechanical condition.

Starters have come a long way and now most are using a gear reduction with more torque delivered for cranking over high compression diesel engines. The Delco 38MT on our Mercedes MBE 900 Diesel engines does the job even with it's light weight. These starters pack a punch and are light weight with mostly aluminum castings.

The planetary gear system in this starter has the typical sun gear (driven by the starter motor) the ring gear is fixed and part of the of the starter body, while the planet gears are part of the actual armature that delivers the starting drive to the engine ring gear.

This set up is extremely durable since there are 3 planetary gears to absorb the load and speed required. I've always been impressed by what a starter can do. It takes a lot just to crank over a diesel engine by hand and when you look at this medium sized starter it's really extraordinary especially when you consider the extra load required for cold starts.

The planetary gear system on this starter has a gear reduction of 3 1/2 : 1 ratio which slows down the output of the drive gear (3 1/2 revolutions of the starter motor to 1 revolution of the output drive gear). What this accomplishes is a high torque output that enables the smaller starter motor to punch out the turning force required to crank over the engine.
Here's a great explanation on planetary gears at "How Stuff Works"
This mechanic information will give you a basic understanding on new starter models that are in use today.


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