Description:1 Title of invention
Sleeve and floating nut arrangement to resolve the shimmy in aircraft nose landing gear.
2 Field of invention
The present invention is related to mechanical engineering in general, and more particularly to the aircraft landing gear system design. The present invention intends to solve the frequently occurring shimmy problem in aircraft nose landing gear system design with fork type arrangement and improve the riding comfort during taxiing, take-off, and landing of the aircraft.
3 Background of invention
Nose landing gear(130) along with a double piston fork (170) arrangement is widely used in civil/military aircraft, as shown in Fig-1. The fork-type design in nose landing gear is embodied in an aircraft that has constraints like limited stowage space, structural constraints in the airframe, and difficulty in of using multi wheel & tyre. With these constraints, the landing gear design has been driven to use fork type (170) of nose landing gear to withstand the vertical load, drag load, and side load.
The wheel (160) and tyre (180) installation, on the fork type(170) of nose landing gear (130),necessitates the requirement of having working clearances between the nose gear piston fork assembly (110) and wheel assembly (140).
To ease the installation of wheel hub(160)andtyre (180), a gap isprovided always between the fork(170) and wheel assembly (140). This gap causes the shimmy problem in the aircraft landing gear system and leads to riding disturbancesduring aircraft ground taxiing,take-off, and landing.
The reason behind the shimmy in fork type of nose landing gearsystem (130) is wheel wobbling, due to the intensional degree of freedom that is createdbetween the piston fork (170) and nose wheel assembly (140) for ease ofinstallation. Various techniques are used in the prior arts, to nullify the shimmy issue and also to enhance lateral stability of the nose landing gear(130). The prior art techniques are: minimising the working clearance between upper toggle(190), lower toggle(200), and middle togglejoint (320);minimizing the degree of rotation of the nose landing gear (130) elements;, and providing appropriate air spring stiffness and designing dedicated damping system. However, the shimmy problem still exists in the nose landing gear (130).
4 Brief Summary of invention
The objective of the present invention is to resolve the shimmy problem with the aircraft nose landing gear system offork-type design.
The objective is accomplished in the present invention by using a sleeve and floating nut arrangement in the fork-type nose landing gear to arrest the lateral movement or wobbling of the wheel assembly (140), that directly nullifies the shimmy issue and also enhances the torsional stiffness of nose landing gear (130).
5 Brief description of drawings
Fig.1 showsatypical nose landing gear (NLG) (130) of an aircraft,comprising various sub-assemblies like upper toggle assembly(190), lower toggle(200), nose wheel steering assembly(100), and barrel assembly(150). The piston fork assembly (110) and wheel assembly (140) are the sub-assemblieswhichare to be installed on the nose landing gear(130), before installing the nose landing gear (130) on the aircraft .
Fig.2 shows the side view and front view of the nose landing gear (130)installation on the aircraft. The figure indicates the nose wheel steeing(100),barrel assembly(150), wheel assembly (140), piston fork (170), and piston fork assembly(110) with piston fork bush(280).
Fig.3shows thewheel assembly (140)and Fig.4 is the section view of Fig.3thatshows the internal partsof the wheel assembly (140).The wheel assembly(140)mainly comprises a wheel hub (160), tyre(180),a plurality of bearing(260),, anda plurality of bearing retainer(270).
Fig.5 shows the exploded view of nose landing gearassembly(130) embodiedwith the present invention to resolve the shimmy issue, comprises of a spacer (230) and a floating nut(220) thatareplaced over a sleeve (250); and axle (240)and axle nut(210)are used for the wheelassembly (140) and piston fork assembly (110).The piston fork assembly (110) is made to hold the wheel assembly (140) in the position as required by the aircraft.
Fig.6 shows thesectional view of the nose landing gear (130),emodied with the present invetion to address the shimmy issue. Thiscomprises thepiston fork (170), wheel hub (160), tyre (180), axle nut(210), axle (240),floating nut(220), spacer(230), sleeve(250),bearing retainer (270), piston fork bush (280), and wire Lock (300).
6 Detailed description of invention
Refer to Fig.1. Typical nose langing gear (NLG)of an aircraft comprises various sub assemblies like upper toggle assembly(190), lower toggle(200), nose wheel steering assembly(100), and barrel assembly(150). The piston fork assembly (110) and wheel assembly (140) are the sub assemblies whichare installed on the nose landing gear (NLG) (130) before it installs on the aircraft.
Refer to Fig.2. It shows the side and front views of the NLG(130) installationon the aircraft. The nose wheel steering assembly(100),barrel assembly(150), wheel hub (160), and piston fork(170) are indicated in Fig.2.
Refer to Fig.3 and Fig.4 for the wheel assembly (140) and its internal parts (160), (180), (260), and (270).
Shimmy is an unstable dynamic phenomenon of lateral and torsional vibration that occurs when a combination of mass, stiffness, and external excitation forces cause a net growth of energy in a dynamic system. Further, shimmy occurs due to insufficient lateral and torsional stiffness of the NLG assembly(130), elastic property of the tyre (180), aircraft mass distribution on the NLG (130), swirling of mass about the NLG (130) shock strut axis, and slackness of the NLG (130) joints.
Shimmy in the fork type of NLG (130) is generally due to wheel wobbling,resulted by the intensional degree of freedom, between the piston fork (170) and nose wheel hub (160), for the ease of installation. Thisgap enhances the level of shimmy in the NLG(130). Various techniques are used to nullify the shimmy issue, such techniques in the prior art are:minimising the working clearance between the upper toggle(190), lower toggle(200), and middle toggle joints (320); minimising the degree of freedom of the NLG (130) joint elements; and providing appropriate air spring stiffness and designingdedicated damping system. ,However, the shimmy problem still exists in the fork type NLG (130) of the aircraft. The present invention of the combined sleeve(250) and floating nut(220) arrangement in nose landing gear (130) design is developed to eliminate the play of the wheel assembly(140) and enhances thetorsional stiffness of the NLG(130), therby reducing shimmy to negligible levels.
The main advantage of using fork(170) type NLG(130) is to achieve a stable load path, utilising minimum storage space and enhancereliability of the nose landing gear (130) system. The fork(170) type of NLG(130)is required in aircraft to have theminimum allowedworking clearance between the piston fork assembly (110) and the wheel assembly(140) for easy installation and frequent removal of tyre(180) due to wear and tear. Excessive wear and tear in the NLG (130) parts are causing slackness, that increases the vibration levels and resulting in pilot discomfort and ultimatly failure of the NLG(130).
Refer to Fig.5. It shows the exploded view of the NLG assembly(130) with the present invention to reduce the shimmy issue. The present inventioncomprises of: a spacer (230) and a floating nut(220) over the sleeve (250), such that the sleeve (250) is inserted into the wheel assembly (140) and the external diameter of the sleeve(250)is designed depending on the inner diameter of the bearing (260);a collar(310) provided over the sleeve (250) that controls the movement of the floating nut(220) in lateral direction; andan axle (240) and an axle nut(210)used for assembly of the wheel(140) and the piston fork (110), such that the outer diameter of the axle (240) is designed to match with the inner diameter of the piston fork (170) and the sleeve (250). The piston fork assembly (110) is used to hold the wheel assembly (140) in the position as required by the aircraft.The floating nut(220)is placed over the sleeve (250). Thefloating nut (220) and sleeve (250) have left handed thread, required to create the intentional gap between the piston fork (170) and the wheel hub (160) during the installation. The interface dimensions of the spacer (230) are made based on wheel installationrequirements.The Piston fork(110) supports the axle(240) that is in turn mounted with wheel and tyre assembly(140) along with the sleeve(250) and the floating nut(220).The axle (240) is used for assembly of the wheel(140) and the piston fork (110). The purpose of the axle (240) is to establish the load path between the ground andthe aircraft structure.The sleeve (250) is inserted into the wheel assembly (140) and at the same time, the floating nut(220) and the spacer(230) are placed on the sleeve(250).The axle nut(210) is used for tightening of the axle(240)with the required torque to take care of the side load of the nose landing gear (130). This is to be done at the final stage of installation. The working clearances between thepiston fork assembly (110) and the wheel assembly(140), for easy installation and making zero slackness, is obtained by adapting the combination of the sleeve(250) and the floating nut (220) arrangement to arrest nose landing gear(130) shimmy problem.
The present invention describes thesignificance of levelof clearances between piston fork assembly (110) and wheel assembly(140) for easy installation and making zero slackness by adopting the combination of sleeve(250) and floating nut (220) arrangement.
Refer to Fig.5 and Fig.6. In the present invention, the following installation procedure is adopted to resolve the shimmy issue in the NLG (130):
i. the wheel assembly (140) is comprising of a wheel hub (160) and a tyre (180)placed in the piston fork assembly(110),the selection oftyre size,tyre stiffness, and charging pressure is based on the load on the NLG(130);
ii. install sleeve(250) with floating nut (220) into wheel assembly (140) on one side and then place the spacer (230)on opposite side of the sleeve(250);
iii. place the total wheel assembly (140)in between the fork assembly(110),insert the axle (240) into the piston fork assembly(110) and then to the wheel assembly (140),ensure that the floating nut(220) is turned till it is butting on the collar (310) before installing the axle (240);
iv. The interface dimensions of the spacer (230) are determined based onwheel hub(160)installation requirements,the spacer (230) inner diametershall be equal to the outer diameter of the sleeve(250),The wheel assembly (140) center shall be aligned with theaircraft center line and then the gap between the bearing(260) andthe piston fork bush (280) on one side is measured, the equivalent gap is to be filled with the spacer(230),based on the measured gap,the functionality of the spacer(230)is to fill the gap and to transfer the side load to thepiston fork assembly(110) during ground maneuvering and drift landing conditions;
v. the axle (240) is used to mount thewheel assembly (140) and the piston fork assembly(110),the axle(240) establishes the load path between ground to aircraft structure through nose landing gear(130),the outer diameter of the axle (240) is designedto match with the inner diameter ofthe piston fork(170)and the sleeve(250);
vi. the sleeve (250) shall be inserted into the wheel(140),the internal diameter of the sleeve(250) matches withthe)outer diameter of the axle (240), the external diameter of the sleeve(250)matches with the inner diameter of the bearing (260), the sleeve(250) is provided with sleeve collar(310)that controls the movement of floating nut(220) in lateral direction; and
vii. the floating nut(220) is placed over the sleeve(250)and hasleft-handed thread tocreate an intentional gap between the piston fork (170)and thewheelhub (160.,this helps in the easy installation of thewheel assembly (140) with the piston fork assembly (110).
The present invention ensures thereduction of shimmy in fork type nose landing gear (130) to a significantly negligible level and provides an easy installation scheme for the NLG (130)of an aircraft. The sleeve(250) is designed with features like collar(310) and thread for the floating nut (220). The floating nut(220) creates the clearance between the wheel assembly (140) and the piston fork assembly (110) during the installation, and also fills the gap between the wheel assembly (140) and the piston fork assembly (110). A wire locking(300)is providedbetween the floating nut (220) and thepiston fork assembly(110) toensurethe positive locking of the floating nut(220) with zero slackness and iteliminates theunturning of the floating nut(220) thatin turn enhances the torsional stiffness of the NLG(130) and minimizesthe shimmy problem.

7 Drawing numerals:

Item. No. Parts Name

100 Nose wheel steering assembly
110 Piston fork assembly
120 Piston fork and wheel assembly
130 Nose landing gear (NLG)
140 Wheel assembly
150 Barrel assembly
160 Wheel hub
170 Piston fork
180 Tyre
190 Upper toggle
200 Lower toggle
210 Axle nut
220 Floating nut
230 Spacer
240 Axle
250 Sleeve
260 Bearing
270 Bearing retainer
280 Piston fork bush
300 Wire lock
310 Collar in sleeve
320 Toggle middle Joint
, Claims:We claim:
1. A sleeve and floating nut arrangement to resolve the shimmy issue in aircraft nose landing gear(130), with a double fork (170) arrangement, comprising:
- a piston fork assembly (110) comprises a piston fork (170) and piston fork bush (280);
- a wheel assembly (140) comprises a wheel hub (160), a tyre (180), a plurality of bearing (260), and a plurality of bearing retainer (270);
- an upper toggle assembly(190) and a lower toggle(200);
- a nose wheel steering assembly (100); and
a barrel assembly(150), charectized in that, a spacer (230) and a floating nut(220) are placed over a sleeve (250) such that the sleeve (250) is inserted in to the wheel assembly (140) and the external diameter of the sleeve(250) is determined based on the inner diameter of the bearing (260); a collar(310) is provided over the sleeve (250) that controls the movement of the floating nut(220) in a lateral direction; an axle (240) and an axle nut(210)are provided to use for the assembly of the wheel(140) and the piston fork (110) such that the outer diameter of the axle (240) is designed to match with the inner diameter of the piston fork (170) and the inner diameter of the sleeve (250); the piston fork assembly (110) is used to hold the wheel assembly (140) in the position as required by the aircraft; the floating nut(220) is placed over the sleeve (250) and the floating nut (220) and the sleeve (250) have left handed thread to create the intentional gap between the piston fork (170) and the wheel hub (160) during the installation; the interface dimensions of the spacer (230) are made on the basis of wheel (140) installation requirements; the Piston fork (110) supports the axle (240) that is in turn mounted with the wheel and tyre assembly (140) along with the sleeve (250) and the floating nut (220); the axle (240) is used for the assembly of the wheel(140) and the piston fork (110); the axle (240) establishes the load path between ground and aircraft structure; the sleeve (250) is inserted into the wheel assembly (140) and at the same time the floating nut(220) and the spacer(230) are placed on the sleeve (250); the axle (240) and the axle nut(210) are used for assembly of the wheel(140) and the piston fork (110);the axle nut(210) is used for tightening of the axle(240)with the required torque to take care of the side load of the nose landing gear (130)that is done at the final stage of installation; the working clearances between the piston fork assembly (110) and the wheel assembly(140) for easy installation and making zero slackness is obtained by the combination of the sleeve(250) and the floating nut (220) arrangement to arrest the nose landing gear(130) shimmy; the sleeve(250) is designed with features like the collar (310) and thread for the floating nut (220);the floating nut(220) creates the clearance between the wheel assembly (140) and the piston fork assembly (110) during the installation and also fills the gap between the wheel assembly (140) and the piston fork assembly (110); a wire locking (300)is provided between the floating nut (220) and the piston fork assembly(110) to ensure the positive locking of the floating nut (220) with zero slackness and it eliminates the unturning of the floating nut(220) that in turn enhances the torsional stiffness of the nose landing gear (130) and minimises the shimmy problem; and the installation procedure to resolve the shimmy issue in the nose landing gear (130) further comprises:
- the wheel assembly (140) that comprises the wheel hub (160) and the tyre (180) is placed in the piston fork assembly(110) and selections of the tyre (180) size, tyre (180) stiffness, and charging pressure are based on the load on the nose landing gear (130);
- the sleeve(250) with the floating nut (220) is installed into the wheel assembly (140) on one side and then the spacer (230) is placed on the opposite side of the sleeve(250);
- the total wheel assembly (140) is placed in between the fork assembly (110), then the axle (240) is inserted into the piston fork assembly (110) and then into the wheel assembly (140), such that the floating nut(220) is turned till it is butting on the collar (310) before installing the axle (240);
- the interface dimensions of the spacer (230) are determined based on the wheel hub(160)installation requirements, the spacer (230) inner diameter is kept equal to the outer diameter of the sleeve(250),the centre of the wheel assembly (140) is aligned with the aircraft center line and then the gap between the bearing(260) and the piston fork bush (280) on one side is measured, the equivalent gap is filled with the spacer(230)based on the measured gap, the spacer(230)fills the gap and transfers the side load to the piston fork assembly(110) during ground maneuvering and drift landing conditions;
- the axle (240) mounts the wheel assembly (140) and the piston fork assembly (110),the axle(240) establishes the load path between ground to aircraft structure through nose landing gear(130),the outer diameter of the axle (240) is designed to match with the inner diameter of the piston fork(170)and the sleeve(250);
- the sleeve (250)is inserted into the wheel(140) such that the internal diameter of the sleeve(250) matches the outer diameter of the axle (240) and the external diameter of the sleeve(250) matches the inner diameter of the bearing (260), the sleeve(250) is provided with the collar(310)that controls the movement of floating nut(220) in the lateral direction; and
- the floating nut(220) is placed over the sleeve(250) and has a left-handed thread to create an intentional gap between the piston fork (170)and the wheel hub (160) that aids the easy installation of the wheel assembly (140) with the piston fork assembly (110).
) and a wire lock(300),