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 Last Update: 28 Oct 2017

- Gearbox Technology -

A detailed presentation covering the technical issues involved in
successful gearbox design and implementation

NOTE: All our Products, Designs and Services are ORGANIC, GLUTEN-FREE, CONTAIN NO GMO's, and will not upset anyone's precious FEELINGS or delicate SENSIBILITIES.

In early 1993, EPI began the development of a lightweight, 500 HP, liquid-cooled V8 aircraft powerplant (see Aircraft V8 Development). Our Propeller Reduction Gearbox ("PSRU") products evolved from that development program. (For those unfamiliar with the term "PSRU", it stands for "Propeller Speed Reduction Unit". We think it's a hideous term, but it has found wide usage.)

The Mark-9 PSRU was the geared propeller reduction unit which we designed, prototyped, and developed for our Aircraft Powerplant. It was also the first of our gearbox products to use hydrodynamic bearings, which substantially reduced the weight of the package. We put the Mark-9 into production in 1997-8. In late 2000, the product was sold to EngineAir Power Systems for use on their turbocharged 440 HP V8 engine package for Lancair-4 and other experimental aircraft.

The design and development of the EPI powerplant (engine, PSRU, and aircraft accessory drives) was the subject of a technical presentation which EPI's CEO gave at the 1996 Advanced Engine Technology Conference (AETC). Since that presentation, EPI has developed a considerable base of technology which supports the design and implementation of reliable, lightweight reduction gearboxes for a variety of aircraft and industrial applications.

EPI has designed and developed several other gearboxes and power transmission systems for general use and for special applications. The MARK-15 is our current, general application high performance, high capacity PSRU.



The success of the EPI PSRU products, plus the help EPI has been able to provide to builders having severe PSRU problems, made it evident that complete, clear information about PSRU’s should be made available. This technology section presents subjects directly related to the proper design of propeller reduction gearboxes driven by piston engines, in an intuitive way, without the need for mathematics. If you are researching the purchase of a PSRU, you owe it to yourself to study these these presentation subjects.

At a bare minimum, you should begin with our "FAQ's" page, which lists the things you MUST know about PSRU's.

Frequently Asked Questions About PSRU's

The COMPLETE Guide to PSRU Technology

 1. Introduction to PSRU Technology An introduction to the subject of PSRU's and why they are necessary.

 2. PSRU's and Torsional Vibration The sources, harmful effects, and solution of the problems caused by the harsh torsional pulsing of a piston engine.

 3. Overview of PSRU Implementations The various approaches which have been taken with PSRU design in an effort to sell inexpensive untis based on some existing product.

 4. PSRU Load Model The concept of a Design Load Model, the severity of an aircraft load model, and the difference between aircraft and automotive load models.

 5. Gear Design and Loads A quick-course in gear design, including such topics as power ratings, basic tooth mesh properties, gear tooth dynamic load, and fatigue of gears.

 6. Superiority of Spur Gears The reasons why EPI uses spur gears for PSRU's, the basic superiority of spur gears, and the large operational history of these gears in high load applications.

 7. Shaft and Housing Loads The sources and severity of the various loads imposed on PSRU shafts, then to the bearings, then into the housings.

 8. Bearing Loads The typical bearing loads encountered in a PSRU and how best to handle those loads.

 9. Lubrication and Cooling The very important subject of PSRU lubrication, and the often-ignored issue of cooling.

10. Prop Governor Drive The effective implementation of a geared prop governor drive.

11. Direction of Engine and Prop Rotation The various tradeoffs which occur when the engine and prop rotate in the same and in opposite directions.

12. Chain Drive Design Issues The Hy-Vo chain drive that EPI designed under contract, and why the typical Hy-Vo Propeller Reduction Drive drive is undesirable.

13. Properties of Belt Drives The fundamentals of belt drives, and performance considerations which are common to all belt drives.

14. One-Way (Sprag) Clutches Insight into one unscrupulous vendor's offerings which contain a dangerous and ineffectual attempt at controlling torsional vibrations.

15. Summary of PSRU Essentials You Should Know A summary of the subjects of PSRU technology you should know when shopping for a reduction unit.

16. Frequently Asked Questions about PSRU's The questions about PSRU technology which we are asked frequently, (which you should ask any PSRU vendor from whom you are contemplating a purchase) and links to the answers.

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