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A tour of the T58 Speed Decreaser Gear (SDG)




The General Electric Speed Decreaser Gear (SDG) for the T58 (series)

Detailed here is the correct General Electric Speed Decreaser Gear (SDG) that originally came as optional equipment on the T58-GE-8F turbine. These massive gearboxes provide a reduction of 3.25:1 and efficiently handle the horsepower of the T-58. Since they are not considered part of the T-58 engine assembly, the SDG usually remains with the airframe when the engine is removed from the aircraft. As a result these units are very hard to locate for sale.

These are the 'as received' photos- it's obviously been sitting for a while, showing signs of exposure to the elements.

I opened the case to clean, inspect, and see exactly what I've got.

Here you can see the arrangement of the gears inside the gearcase. My Maintenance Manual is well written and guides you through the entire inspection process. I was pretty amazed to find no noticeable wear on the bearings or gears.

This is the view inside the cover. The stickers indicate overhaul at NADEP Pensacola during the 3rd quarter of 1993, which coincides with the date the T58 motor this was connected to went into storage. No wonder it shows no signs of wear- it's never been installed on an aircraft since overhaul!

helicopter gearcase

This is a good view of the massive support bearing (Arrow 'A').

This General Electric SDG is equipped with a 'torque sensor'. Actual torque transmitted to the helical gear (Arrow 'B') forces the gears apart. This movement uncovers an oil port, allowing oil pressure to escape to a transmitter, sending a signal to the indicator to display the corresponding amount of torque. This feature is very important as it allows the operator to monitor torque loading so as to avoid overloading the gearcase which is a major cause of SDG failure. (Arrow 'C') shows a scavenge port for lube oil to return to the pump.

(Arrow 'D') shows another lube oil scavenge port. A splash guard keeps excessive oil off the spinning gears (Arrow 'E').

(Arrow 'F' and 'G') show more scavenge ports. A series of lube oil spray nozzles (Arrow 'H') apply oil to the gears to cool and lubricate via supply ports machined into the gearcase (Arrow 'I'). After I receive the O-ring and gearcase sealant I have on order, I'll complete my cleanup and get this reassembled.

After the new seals were installed, a few coats of high-temperature paint were applied and it's ready to install.

This SDG will provide many years of reliable service.

A pair of T58-GE-8 gas turbines with starters and gearboxes is available. Contact me for details.

Here's a pair of General Electric Speed Decreaser Gears ready to install.

This pair is will be included as part of a T58-GE-8B gas turbine package.

This SDG bolts directly to the T58 Power Turbine and reduces the gas turbine output from about 19,500 rpm, down to a usable level of around 6,500 rpm (at 100% power or wide open throttle). Since the Gas Turbine Generator is independent of the Power Turbine, a disk brake can be fabricated to stop the output shaft while the turbine runs. This allows shifting a crashbox, transmission, or effectively provides a 'neutral'.

We have T-58 & Rolls Royce Gnome gas turbines available to power your project.
We also have starters, manuals, instruments, and gearboxes for T-58 and Gnome gas turbine engines.




The Rolls Royce Speed Decreaser Gear (SDG)

The Rolls Royce SDG (Speed Decreaser Gear) was originally produced under license by Bristol Siddeley, until bought out by Rolls Royce in 1966. While the Gnome Speed Decreaser Gearbox is very similar to the General Electric SDG, one of the more obvious differences is the lubrication system. Lube oil is routed through external lines, where the General Electric SDG (shown at the top of this page) is lubricated through internal passages. Either gearbox will bolt up to the GE T58 engine and is suitable for powering boats, tractor pull vehicles or whatever you can install a T58 into.

This unit also features a 3.25:1 reduction. This particular SDG is configured to have the output shaft run forward under the engine. It is also available with a rear output or can be easily modified for rear output and a front shaft brake. These photos show the Rolls Royce SDG with the external lube lines removed.

You definitely don't have a lot of choices when it comes to getting this much power back to the prop. The Arneson ASD8 surface drive and Sage #8 Stern Drive are some of the very few drives rated to handle the tremendous torque and horsepower loads expected from this gearbox. I recently acquired an Arneson ASD8 Surface Drive for my project.

Rolls Royce helicopter gearbox

This Rolls Royce Speed Decreaser Gearbox is setup for 'rear output'.

Here you can see how the external lube lines are routed.

Rolls Royce helicopter gearbox

We have several Rolls Royce Speed Decreaser Gearboxes (SDG) for sale to bolt right up to your T58 Gas Turbine engine. These units come with the correct synthetic gas turbine oil, a copy of the original Factory Maintenance Manual, and are ready to ship now. E-mail me for details

Here's a good view of the external lube system showing the lines, couplings, and sensors on the Rolls Royce SDG. This particular SDG is configured for rear output.

The SDG tucks in real close to the 'cheeks' of the exhaust housing when mounted in place. This SDG has all the lube lines, sensors, and heat shields in place and appears much the same as you would see it when mounted in the aircraft.

The mounting pads provide an easily accessible, solid mounting point from which to fabricate brackets to support the SDG, exhaust housing and the entire rear of the turbine.




Introducing the Rolls Royce 'Coupled Gnome' Gearbox

Some twin engine aircraft like the Wessex 60 helicopter flew twin Rolls Royce Gnome turbines.
In this particular example, a pair of Rolls Royce H1200 Mark 660 turbines were coupled to this gearbox.

This is the Rolls Royce "Coupling Gearbox" Type 100. Either engine (or both) would drive the gearbox with a single (reduced) output.

This gearbox was used in several models of helicopters including the Westland Wessex 60. We have a Combining Gearbox available to power your tractor puller or other turbine powered creation.

This drawing shows the arrangement of the turbines utilizing the Rolls Royce coupling gearbox type 100. This twin engine configuration is also known as the 'coupled Gnome'.

Another view of the gearbox. More details are on my Rolls Royce Gnome page.

Perfect for Tractor Pulls or offshore race boats, this gearbox allows running two Rolls Royce Gnome or GE T-58 gas turbines with a single output.
More details




Please take a look at my "items wanted" list.

My Dream   T58-GE-3   4 Sale   T58-GE-8F   Projects   Gathering   Composite   SDG   Boathouse   History   Gnome   Cobra 30   ASD8   Wanted   Trailer


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