Saturday, November 07, 2009

Starters and Worm Gears in Aircraft Piston Engines

Ever notice how Lycoming and Continental use completely different designs to interface the starter to the engine? We'll focus on the gearing differences between the two.

Continental uses a "starter adapter" for most of their engines that houses a worm gear and clutch mechanism.

Lycoming uses a starter ring gear" mounted to a large "Support Assembly" attached to the crankshaft flange. There are several engineering trade-off's of the two:

  • Lycoming keeps gearing out of the engine. If gear teeth break they don't contaminate the inside of the engine.
  • The heavy Support Assembly might provide some vibration dampening.
  • The starter adapter is compact and light weight.
  • The starter adapter can serve multiple purposes by mounting a pully or dampner to it.
  • The Continental starter adapter has been an expensive item to repair at overhaul (or in between) so Continental engines have one more expense.
  • The Support Assembly is like a large flat plate out in front of the engine that prevents tapering the cowling at the nose.
  • You can mount a pully or pully's to the Support Assembly to drive alternators and vacuum pumps.
  • The Support Assembly can have electrical contacts in the form of circumferal grooves to transfer electricity to the propeller deice system.
  • Both systems have had their own unique problems so it is a trade-off in my opinion as to which one is more reliable. But the Lycoming is less expensive to maintain.

    The fast rotating starter needs to be geared down to the proper starting rpm for the engine. This is where the comparison becomes interesting: Lycoming uses a pinion and gear (small gear on the starter is called a pinion) to achieve the proper gear ratio. Continental uses a worm and gear "worm gear" to achieve the proper gear ratio. Lets compare the two:

 The Lycoming starter ring gear is on the outside and the Contintal worm and gear is on the inside. Notice how compact the worm gear is? Worm gears are often used when large reductions in gearing are needed. In this example, a fast turning starter motor turns the worm. The engine is then turned by the gear - at a much slower rate. Compare this with the large Lycoming starter ring gear that does the same thing but is 2 feet across.

This is a close-up of the Continental worm and gear. Mounted on the back is the clutch spring and drum that disengages the starter when the engine starts. When it works it works well, but there are lots of parts that have the potential of failing and releasing bits of metal into the engine.
The typical worm gear has a brass gear mounted to a steel worm. There's a lot of rubbing motion across the gear teeth so lubricant needs to be continually applied.  The combination of steel on brass prevents cold welding (galling) between the surfaces as lubrication is in the boundry zone where the lubricant isn't always between the surfaces.

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