Inside a Segway PT Gearbox (and all about servicing, elastomers, couplings)

Here is what the beautiful engineering found on the inside of a gearbox from a Segway Personal Transporter (PT) looks like.

Left: inside a Segway PT gearbox. Top Right: the two halves of a gearbox case, with the gearbox seal resting to the side for clarity. Bottom Right: close-up of the hexagonal input shaft, onto which the gearbox coupling is pressed.

The left photo above shows the side of the gearbox that faces the motor, with the helical gears and their bearings on display.

The gearboxes are manufactured at Segway, Inc.’s factory in Bedford, New Hampshire, where each gearbox is assembled, lubricated and sealed. It is designed to require no maintenance over the life of the Segway PT. During development this part was tested for thousands of miles under severe conditions.

Left: Segway motor and a gearbox from an i2/x2 model. The male drive hub on the end of the motor output shaft fits into the female coupling on the input shaft of the gearbox. Right: the other side of a gearbox – the wheel bolts onto the triangular flange attached to the gearbox output shaft.

Under New Zealand conditions, there are already a few first-generation Segway PTs that have travelled well over 30,000 kilometres and still running smooth and quiet on their original gearboxes. Such long life and durability is testament to the fine engineering that has been designed into these gearboxes. As another example, the Segway i2 Patroller deployed at The Base shopping mall in Hamilton by Waikato Security has travelled more than 15,000 km in 3 years, and its gearboxes are still quiet and silky smooth.

To put these distances into context, a private user who travels about 10km every day covers about 2,500km per year. Security guards at Universities, factories and shopping malls who patrol on Segway PTs around New Zealand often cover 20-30km per day. This is two to three times as far as an individual will tend to walk in a daily patrol role, so the productivity benefits of Segway PTs are readily apparent to business users.

We have noticed that the odd gearbox begins to get noisier from around the 10,000 km mark. These have typically been on Segway PTs that are operated in commercial environments where they are subjected to repeated, aggressive start/stop motion (such as Short Ride activities).

Left: Segway motor with drive hub and elastomer. Right: motor, new drive hub, new elastomer, plus a severely worn out elastomer shown for illustration purposes (elastomers should be replaced long before they look like this, otherwise couplings, motors and bearings will likely become worn or damaged).

The only serviceable “wear parts” associated with the gearbox are located outside of the sealed unit. There is an elastomer that absorbs shock loads between motor output and gearbox input, and these are typically checked every 2,000 to 3,000 kilometres – and replaced only if necessary. Often, increased noise from the gearbox area while riding will indicate inspection is needed. For the i2 model, tyres are often replaced around this time, so it can be efficient to inspect the gearboxes, seals and elastomers at the same time. Depending on how the PT model and how it is used and operated, elastomers can last from a couple of thousand to many thousands of kilometres.

Sometimes, the gearbox coupling that attaches to the gearbox input shaft can eventually wear and/or shift off-centre, and this causes the elastomer to wear out more quickly. If this happens, the rider will notice increased noise while riding and/or some “sloppiness” along with a different noise when rocking on the spot or turning left/right back and forth on the spot. It is essential that a worn or un-centred coupling is replaced promptly. Segway NZ has a special jig tool at our workshop that enables us to press a replacement coupling onto the gearbox output shaft.

Left: Segway gearbox with seal visible and coupling fitted, and for illustration purposes another coupling (on its side) plus two couplings standing on end (left). Right: close-up of two couplings standing on end – top coupling is normal while the bottom coupling has its hexagonal-shaped central shaft severely worn out.

In 2003, Segway, Inc’s website described the gearbox as follows:

“The Segway HT’s* gearbox, a joint effort between Axicon Technologies and Segway, is constructed more like a precision Swiss watch than a traditional gear drive from an automobile. A two-stage reduction system provides a 24:1 reduction, allowing the motor to operate at powerful, efficient speeds throughout the full range of speeds of the Segway HT. Each gear is cut to a helical profile, which creates a spiral engagement to minimize noise and increase the load capability of the gears. The number of teeth on each gear is chosen to produce noninteger gear ratios. This means that the teeth will mesh in a different location each revolution, maximizing the life of the gearbox. Our engineers were so obsessed with the details on the Segway HT that they designed the meshes in the gearbox to produce sound exactly two musical octaves apart–when the Segway HT moves, it makes music, not noise.”

The tuned gearbox of the Segway Personal Transporter.

* between 2001 and 2006 the Segway PT was known as the Segway HT – for Human Transporter – as the term ‘PT ‘was in owned by an automotive company in the US and thus unavailable for use by Segway, Inc.

Here is an article about Axicon Technologies developing quiet gears for Segway, Inc. that includes an interview with the founder. And here is more information on helical gears in general, at How Stuff Works.

Helical gears

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