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  Last Update: 25 MAY 2014

- The Superiority of Spur Gears -

As Demonstrated by Use in Hundreds of Thousands of Famous Aircraft Engines

The claim is sometimes made that helical gear trains are more reliable than spur gears. This claim, based in intuition and partial knowledge, often argues the superiority of helicals because of their greater contact ratio.

The typical contact ratio for PSRU-sized spur gears is around 1.6, which means that 60% of the time there are two pairs of teeth in contact and only 40% of the time the entire load is carried by a single pair of mating teeth.

Intuition might lead one to be suspicious of a mechanism in which one tooth-pair carries all the load. That intuition often leads to the claim that helical gears, having a contact ratio in excess of 2.5, would be superior to spur gears (having a contact ratio of slightly over 1.6).

However, most gear engineers know that contact ratio is not the major discriminator between designs. Again, the mathematics and engineering of gear design show that, while helicals do have a greater contact ratio than spur gears of the same pitch diameter and diametral pitch, helicals suffer from the inherent problem of highly-asymmetric tooth loading (edge-loading).

Helical gears do indeed have their uses, especially in cases where quietness is an issue, or where load sharing can be achieved as a by-product of the thrust loads helicals inherently generate. But for the same volume of steel, spur gears are more reliable and considerably less expensive than helicals of comparable quality and size.

There are endless sources of engineering data on the design of reliable spur-gear reductions as well as countless successful applications which rebut the intuitions and the ignorance.

One early self-proclaimed PSRU expert wrote in his newsletter that    "...every spur gear reduction drive would eat up the gears in 10 hours...".    He allowed that    "the Continentals lasted somewhat longer",    but went on to say that Rolls and Allison used "chevron" gears in their reductions (perhaps he meant helical gears, or maybe even the herringbone gears used in the Ranger reduction unit.

This type of claim, unsupported by any facts or science, is representative of the level of misinformation which abounds on the subject of PSRU’s.

(Anyone who doubts that the Rolls and Allison V-12’s used spur gears should make it a point to see one, or at least to look at one of the maintenance manuals. In fact, the very early versions of the Merlin used double-helicals, but the lack of reliability drove Rolls back to a spur gear design.)

The P&W 1830 and 2000 radials had single stage planetary reductions implemented with spur gears. The A and B models of the P&W 2800 had a two-stage planetary implemented with spur gears; the C model used a single stage planetary using spur gears. The P&W 4360 had a single stage planetary which used spur gears (except for the outer teeth on the ring gear which were helical for the specific purpose of implementing a torque-sensor in the gearbox). The Wright 1820 had a single-stage planetary with spur gears.

The Napier Sabre, one of the most amazing piston engines ever built, had a very clever two-stage PSRU which used spur gears in the first stage, followed by an arrangement of helical gears on a huge rocker arm in the final stage. The helicals were used for the sole purpose of implementing true load-sharing between the two crankshafts in the engine.

The Continental GTSIO-520 uses an offset spur gear reduction. Although the engine has only a 1200 hour TBO (recently extended to 1600 hours), that life is primarily a function of the heat load and internal stress imposed by running it at high RPM and at elevated manifold pressure. Unless abused by a ham-fisted pilot, the GTSIO-520 reduction (spur) gears typically last several TBO runs.

In addition to the PSRU's listed above, if you examine the accessory drives of almost all the supercharged WW2 engines, you'll find spur gears almost exclusively, especially in the extremely-high-load application of the supercharger drive units, having step-up ratios of 10:1 to 12:1. These relatively small gears carried several hundred HP driving the blowers and were subjected to unbelievable acceleration loads. For specific examples, see the P&W 1340, 1830, 2000, 2800, and 4360; Rolls Merlin & Griffon; Wright 1820. And for a real surprise, check out the sleeve and accessory drive system on the Bristol Hercules radial (over 25 spur gears).

In addition to widespread use of spur gears in piston engine applications, they are also used in the propeller reduction units in certain turboprop engines. One example is the Garrett TPE-331, which constitutes a large proportion of the installed turboprop population. The TPE-331 with which EPI is familiar has a two-stage PSRU consisting of (a) a small spur gear on the turbine shaft driving a large spur gear on an intermediate shaft which then (b) drives a planetary final reduction which is also implemented with spur gears.

The TPE-330-10, as a specific example, produces 940 Shaft HP at 1591 prop RPM. If you do the arithmetic, you'll find that the propshaft output torque is over 3100 lb.-ft.. However, the turbine shaft turns at about 41,000 RPM, so the torque the turbine produces at 940 SHP is slightly over 120 lb.-ft. In this design, spur gears were the choice for such varying requirements as very high torque (in the output stage) and very high pitchline velocities (over 40,000 FPM in the first stage).

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