Field of the Invention

The invention relates to a belt drive mechanism, and more particularly, to a belt drive mechanism comprising a first flexible member and a second flexible member each having at least one segment with a tensile load of approximately zero newtons during operation.

Background of the Invention

Wind turbines are arranged to capture energy of the wind by means of one or more rotor blades, and to transfer this energy into electrical energy by means of a generator. In some wind turbines, a drive train, including a gear arrangement, is provided for transferring rotational movements of a hub carrying the rotor blade (s) to rotational movements of the generator. The gear arrangement may comprise a number of intermeshed toothed gear wheels which provides an appropriate gearing between the rotational movements of the hub and the rotational movements of the generator shaft. As an alternative, the gear arrangement may comprise a number of pulleys being interconnected by means of a number of belts or chains, in order to transfer rotational movements between the pulleys.

To use a belt to transmit the rotation from the rotor to a generator is known from, among others, WO2015/058770A1. In order to prevent ratcheting or tooth jump, a toothed belt is installed with a preload or tension. The preload must be large enough such that the belt will not jump on the sprocket during full load operation. The preload tension is applied during installation. The preload tension can be a significant source of belt wear and noise. Improper or lack of preload may also cause tooth cracking. It can also diminish system efficiency. Preload for a toothed belt can be over 100 pounds depending belt pitch and width, see Wallace Erickson, Belt Selection and Application for Engineers 277-299, Marcel Dekker, Inc. (1987) .

The prior art relies on a simple routing of the drive belts. Improper allocation of belt tension, routing and alignment will reduce the operational life of a drive belt representing significant cost to repair or replace. It will also reduce the overall efficiency of the turbine drive system, also representing increased costs.

Representative of the art is EP2391825 which discloses a drive device for a windmill comprising a large pulley disposed on a main shaft and at least one belt or chain adapted to transfer rotation from the pulley to a generator. The pulley is rotationally coupled to at least two secondary shafts which are disposed parallel to the main shaft. One or more belts which transfer the rotation, extend over the pulley and the secondary shafts. The secondary shafts are in turn rotationally coupled to at least one, preferably two, electric generators.

What is needed is a belt drive mechanism comprising a first flexible member and a second flexible member each having at least one segment with a tensile load of approximately zero newtons during operation. The present invention meets this need.

Summary of the Invention

An aspect of the invention is to provide a belt drive mechanism comprising a first flexible member and a second flexible member each having at least one segment with a tensile load of approximately zero newtons during operation .

Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings .

The invention comprises a belt drive mechanism comprising a first disc in rotational relation to a secondary shaft, a first flexible member engaged between the first disc and the secondary shaft to rotationally drive the secondary shaft about its axis of rotation, the first flexible member having a segment with a tensile load of approximately zero newtons during operation, a second flexible member engaged between the secondary shaft and an output shaft to rotationally drive the output shaft, the second flexible member having a segment with a tensile load of approximately zero newtons during operation, and the output shaft connectable to a load.

Brief Description of the Drawings

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with a description, serve to explain the principles of the invention .

Figure 1 is a perspective view of the transmission. Figure 2 is a schematic of the drive arrangement.

Figure 3 is a plan view schematic of the drive arrangement .

Figure 4 is a schematic of the first stage drive.

Figure 5 is a schematic of the second stage drive.

Figure 6 is a rear perspective view of an alternate embodiment .

Figure 7 is a front perspective view of an alternate embodiment .

Figure 8(a) is a plan view of a toothed belt.

Figure 8(b) is a plan view of a ribbed belt.

Figure 9 is a front perspective view of the drive schematic as shown in Figure 2.

Detailed Description of the Preferred Embodiment

Figure 1 is a perspective view of the transmission. The transmission is disposed in a typical nacelle frame 400. The transmission comprises first shaft 102 journalled to the frame in pillow blocks 401. Second disc 204 and 205 are each connected to their respective shafts (202 and 203) which are in turn journalled to the frame on pillow blocks 401. Flexible drive member 101 is trained between the first shaft disc and each of the secondary shafts. Rotor shaft 102a is connected to a rotor (not shown) , such as a wind turbine prop. Shaft 102(a) is the shaft for power input to the transmission.

Generator 302 is mounted to the frame 400. Flexible drive member 201 is trained between each secondary shaft disc 204, 205 and the generator sprocket 300. Sprocket 300 is mounted to shaft 301. Generator 302 is the load for the system.

An inventive feature of the system is that a slack side tension for the 1st stage flexible drive member 101 and the 2nd stage flexible drive member 201 are each low, but greater than zero newtons upon installation. Both installation tensions then decrease, tending to zero newtons as full load torque is applied to each stage of the drive. The transmission is a step-up transmission. Most wind turbines rotate from 5 to 20 RPM depending on wind speed. The step up ratio of the instant transmission is about 80 to 1.

Figure 2 is a schematic of the drive arrangement. The first stage drive 100 comprises a flexible drive member 101 that is trained about a first disc 102. A radius Rl establishes an outer perimeter of first disc 102. First disc 102 is attached to a rotor (not shown) such as on a wind turbine. Rotation of the first disc is caused by wind impinging on the rotor blades.

Flexible drive member 101 may comprise a toothed belt, multiple-ribbed belt, flat belt or a chain. The outer perimeter of first disc 102 is toothed, ribbed or flat to engage flexible drive member 101. Flexible drive member 101 may comprise one or more members (a,b,c) mounted side by side in parallel.

Flexible drive member 101 engages shaft 202 and shaft

203. The portion of shaft 202 and shaft 203 that engages flexible drive member 101 is configured as toothed, ribbed or flat to engage member 101. Each shaft 202 and 203 is journalled in pillow blocks 401 or other suitable bearings to allow rotation. The pillow blocks or other bearings are mounted to a frame 400. Frame 400 is mounted in a turbine nacelle (not shown) .

Disc 204 is fixed to shaft 202. Disc 205 is fixed to shaft 203.

Flexible drive member 201 is trained between disc 204, disc 205 and output shaft 300. Flexible drive member 201 may comprise one or more members (a,b,c) in parallel.

Flexible drive member 201 may comprise a toothed belt, multiple-ribbed belt, flat belt or chain.

The portion of sprocket 300 that engages flexible drive member 201 is configured as toothed to engage member 201. Shaft 301 can be connected to a driven load such as an electrical generator 302. The axis of rotation of shaft 301 can be disposed radially outward of the outer perimeter Rl of first disc 102.

Figure 3 is a plan view schematic of the drive arrangement. The first stage 100 is coupled to a rotor (not shown) . The second stage 200 is disposed between the first stage 100 and an output shaft 301 load, such as an electric generator 302.

Figure 4 is a schematic of the first stage drive. System variables for the first stage are identified in Figure 4. A slack side member segment is indicated by Ts¾;0. Since the input torque is split between shaft 202 and shaft 203, there are two belt segments having a slack characteristic at full load, namely, segment 101a and segment 101b. "Slack" refers to having little or no tensile load. In this case the driver is disc 102 and the driven is shaft 202 and shaft 203. Direction of rotation is shown by the arrows in the figures. In an alternate embodiment an electric generator or other load may be directly connected to each shaft 202, 203.

Figure 5 is a schematic of the second stage drive. System variables for the second stage are identified in Figure 5. The slack side member segment is indicated by T3¾i0. Segment 201a has a slack characteristic. For this second stage there are two drivers, 204 and 205. The driven is shaft 301. Direction of rotation is shown by the arrows. Both first stage and second stage rotate in the same direction. The direction of rotation may either be clockwise or counter-clockwise. In this example it is counter-clockwise when viewed from the rotor end (input end) as shown in Figure 4 and Figure 5.

An example solution for the inventive drive follows.

Each of the variables is defined as follows

Symbol [unit] description

θι [ rad] rotation of the shaft 203

ΘΓ [rad] rotation of the shaft 202

θι [ rad] rotation of the disc 102

i [mm] radius of the disc 102

ri [mm] radius of the shaft 203 and shaft 202

Ti [N] tight side tension of drive member 101

Tr [N] tight side tension of drive member 101

Ts [N] slack side tension of drive member 101

Lii [mm] 1st stage drive span length

Llr [mm] 1st stage drive span length

Mi [N/mm] belt modulus belt 101

Wi [mm] 1st stage drive 101 width

Qi [N-mm] torque transmitted by shaft 203

Qr [N-mm] torque transmitted by shaft 202

Qo [N-mm] output torque of shaft 300

θο [ rad] rotation of the shaft 301

R2 [mm] radius of sprocket 205

r2 [mm] radius of sprocket 300

r3 [mm] radius of secondary shaft 202, 203 Ti [N] tight side tension of drive member 201

T2 [N] middle span tension of drive member 201

T3 [N] slack side tension of drive member 201

Lj2t [mm] 2nd stage drive, tight span length of drive member 201

Lj2m [mm] 2nd stage drive, middle span length of drive member 201

L2s [mm] 2nd stage drive, slack span length of drive member 201

M2 [N/mm] belt modulus of belt 201

w2 [mm] 2nd stage drive belt 201 width

Qt [N-mm] torque transmitted between the tight span ΊΊ and middle span T2

Qm [N-mm] torque transmitted between the middle span T2 and slack span T3.

"Symmetric" refers to a system wherein member segments 201a and ΊΊ on each side of the output shaft 301 are of equal length. "Asymmetric" refers to a system wherein segments 201a and ΊΊ are not of equal length.

For a flexible member drive with no member tensioners, also called a "lock center drive", the initial installation tension for flexible drive member 101 and flexible drive member 201, namely, Ts and T3, can be determined in the following manner:

1. A flexible member, in this case a toothed belt for example, is placed between two toothed sprockets of equal size on a test machine. The belt is then subjected to a static pull (load) . Before the load is applied the belt is in a somewhat slack shape. In this state the sprocket hub load versus travel curve is very flat since there is no load on the belt. This is the seating region. Once the belt is seated, the two belt segments between the sprockets become straight as each takes up load. At this point sprocket load versus travel will enter a linear region showing sprocket movement (x) versus load (y) . The transition knee from a horizontal line (no load: y=0) to linear

(loaded) can be easily identified. The tension at the knee region is the required initial tension TO necessary to keep the belt span segments straight. This value is the minimum value of the initial installation tension for the subject belt: T0min. This method can be applied to member 101 and 201.

claims
1. A flexible member transmission comprising:

a first disc (102) arranged for rotation on a first shaft (102a) ;

at least two secondary shafts (202,203) arranged for rotation in parallel to the first shaft, each secondary shaft having a secondary shaft disc (204,205) mounted thereon;

a first flexible member (101) rotationally connecting the first disc to each of the two secondary shafts, the first flexible member having at least one segment with a tensile load of approximately zero newtons during operation; and

each secondary shaft disc rotationally connected to an output shaft (300) by a second flexible member (201), the second flexible member having at least one segment with a tensile load of approximately zero newtons during operation .

2. The transmission as in claim 1, wherein the first flexible member comprises a toothed belt.

3. The transmission as in claim 1, wherein the second flexible member comprises a toothed belt.

4. The transmission as in claim 1, wherein the at least one segment of the second flexible member has an installation tension greater than zero newtons.

5. The transmission as in claim 1, wherein the at least one segment of the first flexible member has an installation tensioner greater than zero newtons .

6. The transmission as in claim 1, wherein each segment of the second flexible member on each side of the output shaft are of equal length.

7. The transmission as in claim 1, wherein each segment of the second flexible member on each side of the output shaft are of unequal length.

8. The transmission as in claim 1, wherein operation is full load operation.

9. The transmission as in claim 1, wherein the first flexible member comprises two or more flexible members.

10. The transmission as in claim 1, wherein the output shaft is connected to an electric generator.

11. The transmission as in claim 1 further comprising:

a third secondary shaft parallel to the first shaft, the third secondary shaft engaged with the first flexible member;

a secondary shaft disc mounted to the third secondary shaft, the secondary shaft disc engaged with the second flexible member.

12. The transmission as in claim 1 further comprising means engaging the first flexible member to cause a wrap angle a of the first flexible member about the third secondary shaft of at least 120 degrees.

13. The transmission as in claim 1 further comprising means engaging the second flexible member to cause a second flexible member wrap angle β about an output shaft sprocket greater than 120 degrees.

14. The transmission as in claim 1, wherein the output shaft is disposed outboard of an outer perimeter of the first disc.

15. A belt drive mechanism comprising:

a first disc in rotational relation to a secondary shaft;

a first flexible member engaged between the first disc and the secondary shaft to rotationally drive the secondary shaft about its axis of rotation, the first flexible member having a segment with a tensile load of approximately zero newtons during operation;

a second flexible member engaged between the secondary shaft and an output shaft to rotationally drive the output shaft, the second flexible member having a segment with a tensile load of approximately zero newtons during operation; and

the output shaft connectable to a load.

16. The belt drive mechanism as in claim 15, wherein the first flexible member comprises a toothed belt.

17. The belt drive mechanism as in claim 15, wherein the second flexible member comprises a toothed belt.

18. The belt drive mechanism as in claim 15, wherein the secondary shaft comprises a first diameter for engaging the first flexible member and comprises a second diameter for engaging the second flexible member.

19. The belt drive mechanism as in claim 18, wherein the first diameter and the second diameter are equal.

20. The belt drive mechanism as in claim 15 further comprising two or more secondary shafts.