From the Shop Floor – Torque Sensing – A Slightly Simplified Explanation

Pos-Neg_Tq_Sense-1The torque sensing system as used in the TPE331-5, -10T and -10 engines installed in Turbo Commanders displays the amount of power being generated by the engine on a cockpit gauge marked either as HP or TQ. While there are several means used in the industry to do this, in this case it is performed with a hydro-mechanical system.

These engines have what is essentially a built-in rotating torque wrench to sense output torque. This gives us the ability to adjust both engines to match power output, set the correct amount of power for a given situation, and obtain the maximum power available without damaging the airframe. So, how is this done, you ask? Let’s examine the system.

The TPE331 engine has two shafts, one inside the other. The larger, heavier shaft supports the compressor disks, the turbine disks, and a special splined spacer along with bearings and seals. It also drives a gear train for the accessories. As with any turbine engine, roughly two-thirds of the energy generated by the turbine section is absorbed by the effort required to spin the compressor disks and accessories. Approximately one-third of the produced energy is what is actually used to drive the load – the propeller.

The smaller, lighter, longer shaft assembly is used to deliver the remaining energy to the prop via a two-stage reduction gear train at a roughly 26:1 ratio (26.228:1 to be specific). At 100% RPM the main shaft is spinning at 41,730 RPM and the prop is spinning at 1,591 RPM. The torsion shaft nests inside the main shaft and connects to it by means of a set of splines at the aft end of the engine. This torsion shaft has a unique characteristic in that it twists a known number of degrees relative to the main shaft as the torque changes.

Inside the engine’s gearbox, the two concentric shafts (main and torsion) together spin the torque sensor. The main shaft is connected to the aft gear and the torsion shaft is connected to the forward sliding gear. This pair of gears is coupled by a set of helical gear teeth such that as the torque changes, the forward gear slides a piston in and out of a sleeve to control a bleed port in the chamber used to vary the oil pressure in a separate measurement chamber.  The same piston also opens and closes a port for the NTS system (but that’s a subject for another time). So much for the mechanical portion of the system.

Now for the system hydraulics, where working fluid is constantly flowing during engine operation. Look at the graphic and follow the path the oil takes from the engine oil pump as it travels to the positive torque pressure regulator. This regulator drops engine oil pressure to a specified value. The pressure-regulated fluid then travels through a calibrated orifice into a measuring chamber.

As the torque produced by the engine varies, the torque sensor piston varies the opening in the bleed port. It closes the bleed opening as more torque is produced, and opens the bleed port when less torque is produced. So, the pressure in the measurement chamber is controlled by the amount of leakage created by the torque sensor assembly.

In the sketch you’ll notice a plug depicted below the orifice assembly. This is a historical artifact from the days when a “hydraulic compensator” was used; it has since been replaced by an adjustable electronic pressure transducer. In new-production gearbox cases this plug has been removed because it is no longer of use, and removing it simplifies the gearbox casting.

So now we have a working fluid pressure that changes in direct proportion to the output torque of the engine. We need to translate this pressure to something we can use in the cockpit. And, because this engine design operates with a slight vacuum in the gearbox, we also need to take that vacuum into account for the system to indicate correctly.

The gearbox housing has two dedicated ports for this purpose. One port provides the torque pressure, and the other provides the case vacuum pressure. Both ports are connected to an adjustable, electronic differential pressure transducer. The transducer in turn provides the electrical signal necessary for the cockpit gauge to indicate the engine’s power output.

The same signal also is used by the HP/TQ limiting system to prevent overloading the airframe. This electronic transducer is calibrated to provide a +5 VDC signal at what is idle torque, and a +/-0 VDC signal at 100% torque. The signal also goes negative VDC in an over-torque condition – but that’s another subject.

As a side note, does your HP/TQ gage display 100% HP/TQ when power is off, and 0 HP/TQ when aircraft power is turned on with engines off? It should. If it doesn’t, you should get with your favorite Twin Commander Service Center and have it corrected.

To calibrate this torque sensor, the engine is operated with a very expensive, very sensitive, specially calibrated electronic strain gauge type torque sensor mounted between the engine’s output shaft and a load prop. (You’ve probably heard mechanics talk about “the Lebow,” the strain-type torque sensor used for this task.) The engine is warmed up to get the oil to its normal operating temperature, then it is operated at various conditions (a set of specific torque values), followed by a specified torque output value representing the 100% power value for that engine/airframe combination.

The engine torque sensing system pressures (torque signal and case vacuum) at those two data points are noted and recorded on the DSC sheet (Data Sheet Customer) for that engine. Next, a torque transducer is connected to a voltmeter to monitor the output signal, and it is powered and connected to a variable, calibrated, pressure source. It is carefully adjusted to indicate the correct torque at the pressures for the various specified conditions and to indicate 100% torque at the pressure for that point. As a result of this careful calibration the transducer and engine are mated together. If the transducer is to be removed and mated to another engine, it has to be re-calibrated to match that particular engine/airframe combination.

So there you have it – a clever way to give you an indication of the amount of power your engine is providing. Of course, you also use other instrumentation to monitor the engine. And please note that this is not “dial a torque.” The torque transducer output voltage must be adjusted to the values specified in the DSC sheet for that particular engine and airframe combination. If not, you will get a false indication in the cockpit and possibly an incorrect torque limiting system operation.

Mike Grabbe has been in the business of maintaining aircraft since 1970. He worked for a couple of FBOs, and was a Field Service Rep for a turbine engine manufacturer as well as a factory service rep for the Aero Commander Division of Gulfstream Aerospace. He instructed for a number of years at an FAR 147 school, and prior to working at Eagle Creek Aviation Services was the Director of Twin Commander Maintenance Training at FlightSafety International in Bethany, Oklahoma.