1 Title of the Invention

Mechanically Closed Loop Test Rig for Helicopter Tail Gear Box Testing

2 Field of the Invention

This invention finds application in ground testing of helicopter gear boxes specifically the simulation of tail rotor loads that are dynamic in nature at operating power. It is basically a test rig in a mechanically dosed loop foursquare type arrangement.

3 Background of the Invention

The transmission components and system transfer power from engines to the main and tail rotor systems of the helicopter at specific speed and direction. Transmission system components undergo loads which are dynamic in nature and required to deliver high performance and reliability during service. Intended functions and the design loads on the components of the helicopter gearbox system are simulated under dynamic conditions on a test rig that is comprised of mechanical, hydraulic and electrical driving and loading devices and are controlled, monitored and recorded continuously during the tests. The test rig disclosed here in this article is for testing of gearbox that is under development/acceptance to validate the design for the intended performance.

3.1 Prior Art

US patent US4615212 discloses of testing components such as gears, clutches and universal joints includes a pair of spaced transmissions interconnected to each other through a pair of connecting links to form a closed mechanical energy circuit. A coaxial twist coupling which is rotatable relative to the gear.

For torque application type used in the loop is twist coupling with hydraulic system and vanes, a twist coupling is for applying controlled torque loads to the specimen.
U.S. Pat. No. 2,981,103 issued Apr. 25, 1961 to W. G. Livezey, uses two parallel lines whereas in our application there is are three gearboxes, one specimen gearbox and four shafts to form closed loop lines not necessarily parallel.

German patent document E-OS No. 1573 682, is used for testing of shafts with offsets and not gearboxes. For example, the torque applier of U.S. Pat. No. 2,981,103 included multiple planetary gear sets, clutch assemblies and brake assemblies. In devices of this nature, it was inherently difficult to provide rapid changes in direction of applied torque, rapid speed changes, and other true load conditions in simulation of actual usage. In addition, the induced torque load from the torque applier to the test specimen in U.S. Pat. No. 2,981,103 was transmitted through a pair of pinions, a pair of shafts and a torque meter. By providing such indirect and lengthy transmission route, torque loads in true simulation of actual conditions were difficult to achieve and the possibility of twist deflection occurring in the route between the torque applier and the test specimen was present.

US patent US4294112 provide an improved four-square locked-in static torque machine for testing splined connector parts. This invention to provide a test machine to test one or more sizes of splined connectors at the same

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time, and capable of simulating a range of shaft misalignments. Manual lock-in torque, only two gearboxes in the mechanical loop, shafts pairs are to be parallel. For testing mating splined parts under rotational torque load and axial misalignments A four-square test machine to which a locked-in static torque is applied to simulate actual loading on splined connector parts during rotation.

The principle of this type of machine is disclosed in U.S. Pat. No. 3,078,711, wherein gears and shafts are formed in a loop to define a four square into which static torque is applied. The contribution made by the disclosure of this patent is that the machine at rest is under no load, and torque is applied
progressively as rotational speed increases to simulate actual drive conditions.

U.S. Pat. No. 3,112,643 provides a four-square test machine having capability of providing torsional input to the loop while the machine is rotated. The idea of providing apparatus for varying the locked-in torque is disclosed in U.S. Pat. No. 3,195,350. It is capable of applying torque loading in either direction such as would be experienced in driving certain types of machinery. It is known, such as in U.S. Pat. No. 3,796,092, to apply lateral forces to test specimen under simulated actual working conditions.

US patent US3690168, a four-square test device, shafts connected into a pair, and each pair comprises two shafts connected in series with universal joints with two torque loading channels for each shaft line. Mechanisms is used her is between the parallel members of the test device. And comprise a system where generally four substantially identical specimens, application is limited to shafts or axles in a system.

Torque applied here is, one pair of shafts is connected to the other pair with chains or gears so that the shafts can be torque loaded against each other. The torque in one pair of shafts is opposed by the torque in the other pair of shafts through the gears. The amount of torque in the shafts of course depends upon the position of the gears. It is also known to introduce dynamic torques into this type of test system through the use of an actuator acting on one of the sides of the system only, and this increase or decrease in torque had to act through the connecting gears on the opposite end of the series coupled specimen to be transmitted into the parallel specimens.

In this arrangement each gearbox used here should have similar gear pair, in our invention gear pair depends on gear ratio. Torque is controlled through two vane type rotary hydraulic actuator with servo. Here test specimens to be subjected to torque arranged in a series-parallel system comprising two sets of specimens.
4 Brief Summary of the Invention

This invention disclosed hereunder is about a test rig that is used for testing of helicopter tail rotor gear box. Testing is for various needs like for obtaining the gears tooth contact pattern, for functional checks of the integrated gearbox and also for loading gearbox components to establish their capability while evaluating the overall performance of the gearbox.

The test rig is based on the principle of mechanically closed loop system (Four Square Principle) wherein power from the output Tail Rotor Shaft (11) of the test specimen gearbox (50) is fed back to the input shaft assembly (40) of the test gearbox (50) via rig gearboxes (30, 70, 80), mast loading block (60) and inter connecting shafts (20,40). However, there are frictional losses in the loop which are overcome by the power supplied by the main drive motor (10) to the loop.

Thus, the test becomes economical due to the power required is small as compared to the full-load test power of the test specimen. Power consumption to run the rig is in the order of about 10% of the total power required for the gearbox testing.

The test rig frame design facilitates easy installation of all gear boxes to be tested, supporting structure for rig gearboxes is assembled such that it does not hinder the working area during assembly and dis-assembly of Test specimen on the test rig. Overall, the structure is able to withstand the test specimen loads with minimum deflection and vibration.

5 Detail Description of the Drawings

Figure 1: Detailed construction of test rig which shows Base structure (1) with shock/vibration mounts (2), drive Motor (10) with mount (3), rig gearboxes (30, 70,80), mast loading unit (60), universal shaft (20), flexible drive shaft assembly (40), test specimen gearbox assembly (50) all are in mechanically closed loop arrangement.
Figure 2: schematic layout of mechanical closed test rig disclosed in Figure

2.

Figure 3: Rack (91) and Pinion (92) arrangement used in the swivel torque loading system (90) for torque loading.

Figure 4: Shaft integrated with universal shaft (20), which is used to establish the connection between the gearboxes (30,70,80) and mast loading block (60).

Figure 5: Shaft integrated with the flexible coupling, which is used to establish the connection between the specimen gearbox (50) and gear box (30).

6 Detail Description of the Invention

This invention disclosed hereunder is about a test rig that is used for testing of helicopter tail rotor gear box. Testing is required for various needs like for obtaining the gear mesh tooth contact pattern, for functional verification of the integrated gearbox and also for loading, gearbox components to establish their load bearing capability while evaluating the overall performance of the gearbox. It is basically arrangement based on foursquare or mechanically closed loop principle. Also, the present invention is not limited to gearbox and it can be used for any type shaft to test in this set up within the scope of the disclosure made hereunder.

Referring to Figure 1, the test rig disclosed in this article consists of a base structure (1), which is composed of frames (1a, 1c) and columns (1b) to facilitate the installation of test rig gearboxes (30,70,80), mast loading block (60) and test specimen (50). The test rig structure (1) made with base frames (1a, 1c) and columns (1b) welded, which have flexural and torsional rigidity to control the deformations of the test rig to prevent the misalignment of shafts when loaded. Test rig further consists of a motor mount structure (3) to mount the drive motor (10). Motor mount structure (3) consists of frames (3a,3c), columns (3b) welded together to enable the driver motor
(10) mounting. The motor mount structure (3) and the base structure (1) are further connected through interface beam plate assembly (8) to ensure the sufficient integrity between them. Both motor mount structure (3) and the base structure (1) are mounted onto a strong concrete floor through shock absorbing mounts (2) to absorb structural shocks/vibration which are generated during the test.

Shock mounts (2) are used to absorb vibration from the frames and columns (1a,1b,1c, 3a,3b,3c) of the base structure (1) and motor mount structure (3) and also to relieve the stresses that may be induced from the loads while test rig is running which means the shock mounts work like vibration isolators to make the test rig self-reacting.

In order to form mechanical closed loop system, the gearboxes (30, 50,70,80) are arranged in series. The rig gearbox -1 (30), rig gearbox-2 (70), rig gearbox-3 (80) integrated with Torque loading system (90) and the test gearbox (50) with a tail rotor shaft (11) are positioned and assembled on respective mounting fixtures (4,5,6,7) installed on the base structure (1). For mounting all the gearboxes, the fixtures are made by welding MS plates and fastened to the base structure (1) to form a complete structure.

The motor (10) is connected to rig gearbox-1 (30) with the help of a shaft (20) integrated with universal coupling on either ends, gearbox-1 (30) connected to test gearbox (50) with a shaft assembly (40) integrated with flexible coupling on either ends, specimen gearbox (50) with a tail rotor shaft

(11) connected to a mast loading unit (60) through a flanged coupling (12), mast loading unit (60) and gearbox-2 (70) connected through a universal shaft (20) integrated with universal coupling on either ends, gearbox-2 (70) and gearbox-3 (80) connected by a shaft (20) integrated with universal coupling on either ends and gearbox (80) and gearbox-3 (30) connected by means of a shaft (20) integrated with universal coupling on either ends to close the loop mechanically. The use of shaft (20) with universal coupling and the shaft assembly (40) with flexible couplings at the ends of the shafts
that are interconnecting the rig gearboxes to take care of possible misalignments between the shaft axes of the gearboxes.

The motor (10) supplies power to rotate the entire mechanical loop transmission (20,30,40,60,70,80) system and with test specimen gearbox (50) at rated speeds and direction of rotation.

The motor (10) output shaft is aligned with input shaft of gear box (30). The gearbox (30) output shaft is aligned with input shaft of test gearbox (50). Gearbox (50) output shaft is aligned with input shaft of gearbox (70), gearbox (70) output shaft is aligned with input shaft of gearbox (80). The gearbox (80) output shaft is aligned with input shaft of gearbox (30) in return and all the gearboxes are interconnected through a pair of connecting links (20-40-20-20-20) to form mechanically closed loop. For measurement of Torque and speed a sensor (100) is connected at the interfacing line of test gearbox (50).

The sequence of the mechanical closed loop consists a motor (10), universal shaft (20), rig gearbox-1 (30), flexible drive shaft assembly (40), test specimen (50) with a tail rotor shaft (11), flanged coupling (12), mast loading assembly (60), universal shaft (20), rig gearbox-2 (70), universal shaft (20), rig gearbox-3 (80) and universal shaft (20) back to rig gearbox-1 (30).

Referring to the Figure 4, universal shaft (20) assembly that is used in this embodiment is comprised of a shaft (24) integrated with a male spline (25) on one end and a flange (21) on the other end of the universal coupling (22); similarly, a shaft (23) with female spline on one end and a flange (21) on the other end. The same universal coupling is being used to connect the gearboxes as disclosed in this article to alleviate the misalignments.

Referring to the Figure 5, shaft assembly (40) used in this embodiment is comprised of a shaft (43) assembled with a flexible coupling made of laminated sheets (42) and a flange (41) fitted on the both the ends of the
shaft (43). Shaft (43) is further integrated with an inbuilt ring (44) to remove the un balanced mass in the shaft to minimize the vibration.

Referring to the Figure 2, it shows typical mechanically closed loop layout in which the basic requirement of mechanical closed loop is that the gear ratio of complete closed loop system should be unity and direction of rotation to be followed.

For driving of mechanical closed loop system, a drive motor (10) is being used. The drive motor (10) is positioned and assembled on motor mount fixture (3).

Rig gearbox-1 (30) is a two-stage gearbox consisting of one bevel gear stage, number of teeth on gear (T9) and number teeth on pinion (T10).and one spur stage, number of teeth on pinion (T11), number teeth on gear is (T12).

Test specimen gearbox (50) consisting of one spiral bevel gear stage number of teeth on pinion (T1), number of teeth on gear (T2).

Test rig gearbox-2 (70) consisting of one bevel gear stage number of teeth on pinion (T3) and number of teeth on gear (T4).

Rig gearbox-3 (80) is a two-stage gearbox consisting of one bevel gear stage, number of teeth on pinion (T5) and number teeth on gear (T6) and one epicyclic/planetary gear stage, number teeth on ring gear (RG) is (T7) and number of teeth on sun gear(S) is (T8). Planet gear (P) is attached with planet carrier (PC), it has a stationary member of planetary gear system and it is integrated with Torque Loading System (90).

Rig gearboxes have to be designed to form a mechanical closed loop in such way that the gear ratio and direction of rotation has to be followed by mechanical closed loop concept. Therefore, the selection of number of teeth on gear and pinion of each rig gearbox to get required unit Gear ratio for functioning in mechanical closed loop is;
T1 T3 TS T7 T9

Gear Ratio = — x — x — x — x —— = 1 T 2 T 4 T 6 T8 T10

Further referring to the Figure 4, towards loading of the test gearbox (50), a swivel torque loading system (90) is integrated with rig gearbox-3 (80). The torque loading unit consists of a rack (91) and a pinion (92) gear arrangement and this mechanism is used for conversion of linear to rotary motion. Number of teeth on rack (91) is (T14), number of teeth on pinion (92) is (T13). Stationary planet carrier (PC) of the loading gearbox (80) is connected with pinion shaft (92) of the torque loading unit (90) by means of spline/keyway. Torque is applied by way of creating rotary motion of the pinion (92) in opposite direction of the ring gear (RG) of the rig gearbox-3 (80). Hydraulic power is supplied to the desired port F1 or F2 of rack piston. (91) based on the requirement of pinion (92) direction (93) and perpendicular distance between axis of rack (91) and pinion axis (92) is R. The swivel torque loading unit is electro hydraulic operated.

Swivel torque loading system (90) is flange mounted on to the loading gearbox housing (80) with the torque transfer connection through a stationary planet carrier (PC) having hollow shaft with internal splines or . keyway, for the application of load on the specimen gearbox (50) during the test runs.

Torque loading method is not limited to rack (91) and pinion (92) gear arrangement but also possible to use Epicyclical Gear Train within the scope of the invention disclosed in this embodiment.

For test specimen gearbox mount is design such that the assembly on to the test rig to be similar as per actual mounting position on the helicopter with respect to x-y-z co-ordinate system of Helicopter and also the mounting angles on the helicopter at which the tail rotor gear box is installed.

The particulars disclosed in this article are exemplary representations of the invention, it is also possible that the invention may be modified and practiced in equivalent but different ways or arrangements by one who is
skilled in the art with the help of the details described in this manuscript. Therefore, all such variations are to be considered within the scope and spirit of the invention disclosed in this article. Accordingly, the protection is sought.