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.

Inspecting High Strength Materials

The Aardvark Syndrom - built strong but easy to break

Drop a glass onto the floor and it shatters, drop a block of wood onto the floor and it doesn't - yet both have approximately equal tensile strengths. A high tensile strength steel bolt might be twice as strong as a mild steel bolt yet it fractures in two whereas the mild steel bolt bends but still holds the structure together.

We learn to handle glass objects differently than wooden or metal objects because "they break easily"and "are brittle." For the same reason if we use brittle materials in structures we need to 'be careful" and provide additional protection to avoid breakage. A high tensile strength steel bolt might be 3 times stronger than a mild steel bolt but it takes 10 to 100 times less energy to break!1.

A mild steel bolt can handle small nicks, or a little bit of corrosion pitting not because it is strong but because it is hard to fracture (crack). Even if it does crack, the crack growth is so slow that we use various NDT methods to detect cracks and replace the bolt before it breaks.
Not our high-tensile steel bolt. The energy required to break it is 10 to 100 times less. A bit of corrosion that creates a pit that concentrates stress might be all it takes to start a crack. Because it doesn't take much energy to grow the crack, the part may fracture as fast as a broken glass.
There are various methods of using "high-strength-low fracture energy materials, such as better envirnomental protection, non-critical applications, redundent load paths, crack arresting structures. But using a high tensile steel bolt ("Grade 8") as a single point of attachment on a trailer hitch that is bathed in road salt and submerged in lakes where corrosion occurs hidden under the head or shank is not one of them. A better idea would be to match strength with fracture energy!

More inspections are required for high-strength low fracture energy materials:

• More frequent inspections for corrosion.
• Protection from scratches and marks.
• Protected tooling that won't mar the surface.
• More frequent application of corrosion inhibitors.
• More adequate and detailed inspection and rejection instructions.
• Increased education of mechanics and their employeer on why this is so.

For procurement, better quality audits of the manufacturing process, as these parts require more precise materials , process, and heat-treat.

Note 1. Approximate work of fracture J/(m squared) for mild steel is 100,000 to 1,100,000. For high tensile strength steel it is approximately 10,000.

Fingerprint Corrosion on Aircraft Products

Many years ago our aircraft hose manufacturing shop http://www.sacskyranch.com/ set up a program to identify and eliminate products that might cause chloride contamination. Every chemical introduced into the area is screened for chloride or other potential corrosive materials. This program is not a bad idea for any repair facility working on advanced aircraft products. What we couldn't eliminate was the human touch and the secretions deposited onto surfaces. Fingerprints cause corrosion, and police are using it to identify individuals who have touched brass cartridges years ago:

"We recently showed how fingerprints on brass cartridge cases that we left out for several days in open air at room temperature can still produce corrosion sufficient for visualization, even after they have been washed in warm water and detergent to remove the residue"

Our concern is corrosion pitting that might damage a critical aircraft part. So what can you touch and what should you be careful with? How do you clean fingerprints from parts?  read more on my web site...

Slick magnetos and propeller strikes

 Hi John, a couple of us mechanics have been talking about what a Slick mag needs after a prop strike, I've been on the Unison website and can't find any info, where could I look for something in print?


I am not aware of any Slick recommendations for inspection after a propeller strike. Of course, Champion/Slick would be the people to ask.

In my experience of doing propeller strike inspections (approx. 12 per year for 10 years), magnetos were never part of the inspection. This was before Continental/Bendix added their magneto inspection requirement. My recollection (it is often faulty at this age), is that they blamed distributor gear tooth breakage in the magneto on sudden stoppage/propeller strike. Picture below is the distributor gear with broken teeth:

Our opinion is that this breakage is caused by worn bushings creating a conical oscillation about the center axis (whirl or shopping cart wheel flutter). This is a very rare event but we have witnessed it on a magneto test bench.

For more opinions on propeller strikes:

Should an engine be torn down after a prop strike?