Conveyor drive system and motor built-in reducer therefor

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to conveyor drive systems, in particular, to a conveyor drive system with lowered vibration, noise, or pulsation, and a motor built-in reducer used for the conveyor drive system.

2. Description of the Related Art

FIG. 9A

is a top plan view and

FIG. 9B

is a front view of a prior art conveyor drive system, in which the reference numeral

100

denotes a conveyor and the reference numeral

200

denotes a drive system therefor.

The conveyor

100

includes a bed

101

, a pair of drums

102

rotatably supported at both ends of the bed

101

, and an endless conveyor belt

103

trained over both drums

102

. The drawings illustrate only one end of the bed

101

and one of the drums

102

. A driven sprocket

104

is coupled to the shaft of the illustrated drum

102

, so that the drum

102

is driven by a geared motor

201

via a chain

105

trained over the driven sprocket

104

. The reference numerals

106

,

107

, and

108

in the drawings represent an object being transported, rollers, and a support leg, respectively.

The drive system

200

includes the geared motor

201

having an output shaft

222

to which a drive sprocket

203

is mounted. The endless chain

105

is trained over this drive sprocket

203

and the driven sprocket

104

of the conveyor

100

. The reference numeral

204

represents a base. The support legs

108

of the conveyor

100

are joined to this base

204

.

The geared motor

201

consists of a motor

210

and a reducer

220

. The reducer

220

may be, for example, a planetary gear reducer disclosed in Japanese Patent Laid-Open Publication No. Hei. 8-4844, a parallel shaft gear reducer using helical gears, or an orthogonal gear reducer using hypoid gears proposed by the applicants of the present invention disclosed in Japanese Patent No. 2628983. In the applications where a low noise, low vibration environment is particularly desirable, it is the practice to use a reducer employing a helical gear set or an orthogonal gear reducer employing a hypoid gear set.

For example, in the application as an inspection conveyor for the visual checking for any small foreign matter in a food product package, it is highly desirable to suppress vibration or pulsation of the conveyor belt as much as possible.

While the demand for such conveyor has been growing, no improvements have been made so far that address this problem.

SUMMARY OF THE INVENTION

The present invention has been devised under these circumstances, taking account of the problems caused by vibration, noise, or pulsation in conveyors. An object of the invention is to provide a conveyor drive system and a motor built-in reducer used for the system, by which problems resulting from vibration, noise, or pulsation in conveyors are eliminated.

To solve the above problems, the present invention provides a conveyor drive system including a motor, a reducer mechanically interconnected to the motor for transmitting rotation of the motor at a reduced rate to a drum for driving the conveyor, and a transmission unit for discretely transmitting output from the reducer to the drum. The transmission unit includes a driving rotary member, a driven rotary member, and an endless power transmission member trained over the driving rotary member and the driven rotary member, for discretely transmitting the output. The reducer is a simple planetary roller reducer having a sun roller, a plurality of planetary rollers in rolling contact with an outer periphery of the sun roller, and a ring roller, the planetary rollers being in contact with an inner periphery thereof. An output shaft of the reducer is connected to the driving rotary member of the transmission unit.

In known conveyors, generally, when one of the driving and driven sprockets (the driving rotary member and the driven rotary member) in the endless power transmission mechanism has too small a pitch circle diameter, the links of the chain trained over the sprockets (transmission unit) do not form a smooth arc but numerous sides of a polygon. As a result, the chain moves around the driving and driven sprockets as if discretely, making a rattling noise. This movement of the chain will be hereinafter referred to as a “polygonal motion” throughout this specification.

Such polygonal motion is most likely to occur in power transmission using a metal chain and sprockets, but it also can occur in a belt drive using a rubber or plastic timing belt. That is, for either a chain drive or a timing belt drive, as long as power is discretely (digitally) transmitted, the polygonal motion occurs in these types of power transmission. This phenomenon has not been considered a problem so far.

Recent use however of an inverter power supply with converting frequency which enables a variable speed drive of a motor has brought about a problem of unexpected noise from not only the chain but also from the geared motor and its neighboring constituents such as a bed on which the geared motor is placed, or supporting legs and bases. This noise is apparently caused by resonance between the chain or timing belt and the driving geared motor, which occurs at certain rpm of the motor.

The inventors have ascertained through research that this problem is likely to occur particularly when the chain sprocket has a small pitch circle diameter.

This finding indicates that the resonance occurs in association with the above-described “polygonal motion.”

It is generally easy to take measures to suppress noise and vibration if the level of the noise and vibration is constant. It is however, not easy to take appropriate measures to prevent noise or vibration which is generated unexpectedly depending on the situation. This is because it is not desirable in terms of cost to provide measures taken to prevent such unexpected noise or vibration, which result in excessive quality during the majority of the operation period.

In the case where the resonance is presumably caused by the polygonal motion, the problem could be solved to some extent by increasing the diameter of the sprockets. However, it is possible that objects being transported on the conveyor belt are positioned beyond the conveyor belt edge. Therefore if the sprocket is to have a larger diameter, then the drum itself or the entire conveyor must be made large in order that the sprocket does not stick out from a side of the belt. These measures however increase the equipment costs and require more space to install the system. Another option to prevent the “polygonal motion” is to employ a flat smooth belt instead of a chain or a timing belt to perform analogue or seamless power transmission using friction.

However, the structure of the analogue power transmission does not allow a large torque to be transmitted, and accordingly the overall mechanism tends to be bulky. Also, the problem of slippage at the belt-to-pulley interface which is intrinsic to this mechanism makes it hard to perform feed-forward or feed-back control, and therefore precise positioning or control of speed is impossible.

The present invention presupposes the use of chains or timing belts and aims at solving the problems described above. The primary feature of the invention is that it employs a reducer of a simple planetary roller mechanism which performs traction transmission for the reducer interposed between the motor, which is the source of vibration and thus the source of resonance, and the chain or timing belt which cannot help but generate a polygonal motion. Data proving the effects of using traction transmission will be given later.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A

is a top plan view and

FIG. 1B

is a front view of a conveyor and its belt drive system according to a first embodiment of the present invention;

FIG. 2

is a cross section of a planetary roller-type, motor built-in reducer of the conveyor belt drive system;

FIG. 3

is a cross section taken along the line A—A of

FIG. 2

;

FIG. 4

is a graph showing the results of an experiment in which the present invention is compared to a prior art conveyor and its belt drive system;

FIG. 5

is a graph showing the results of another experiment comparing the present invention to the prior art example;

FIG. 6A

is a top plan view and

FIG. 6B

is a front view of a conveyor and its belt drive system according to a second embodiment of the present invention;

FIG. 7

is a cross section of a conveyor and its belt drive system according to a third embodiment of the present invention;

FIG. 8

is a cross section of a conveyor and its belt drive system according to a fourth embodiment of the present invention; and

FIG. 9A

is a top plan view and

FIG. 9B

is a front view of a prior art conveyor and its belt drive system.

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be hereinafter described with reference to the accompanying drawings.

FIG. 1A

is a plan view and

FIG. 1B

is a front view of a conveyor and a conveyor belt drive system according to one embodiment of the present invention, in which the reference numeral

300

denotes the conveyor and the reference numeral

400

denotes the drive system.

The conveyor

300

includes a bed

301

, a pair of drums

302

rotatably supported at both ends of the bed

301

, and an endless conveyor belt

303

trained over both drums

302

. The drawings illustrate only one end of the bed

301

and one of the drums

302

. A driven sprocket

304

is coupled to the shaft of the illustrated drum

302

, so that the drum

302

is driven by a motor built-in reducer

401

(to be described later) via a chain

305

trained over the driven sprocket

304

. The reference numerals

306

,

307

, and

308

in the drawings represent an object being transported, rollers, and a support leg, respectively.

The drive system

400

includes a planetary roller-type, motor built-in reducer

401

having an output shaft

422

to which a drive sprocket

403

is mounted. The endless chain

305

is trained over this drive sprocket

403

and the driven sprocket

304

of the conveyor

300

. The reference numeral

404

represents a base. The support legs

308

of the conveyor

300

are joined to this base

404

.

As can be clearly seen from

FIGS. 1A

,

1

B,

9

A, and

9

B, the drive sprocket

403

mounted to the output shaft

422

of the motor built-in reducer

401

and the driven sprocket

304

in the embodiment of the present invention are much reduced in their diameter compared to the prior art example. There is thus no risk of sprockets interfering with the objects

306

being transported. Since the drums

302

are also reduced in diameter, the entire conveyor

300

is made smaller and thinner.

Such reduction in the diameter of the drive sprocket

403

, driven sprocket

304

, and drums

302

can only be achieved by the application of the planetary roller mechanism for the motor built-in reducer

401

.

FIG. 2

is a cross section of the planetary roller-type, motor built-in reducer

401

constituting the conveyor drive system of the present invention, and

FIG. 3

is a cross section taken along the line A—A of FIG.

2

. The reduction gear set

401

comprises a motor

410

and a simple planetary roller-type reducer (hereinafter referred to as a reduction gear unit)

420

. Rotation of the motor

410

is transmitted to the reduction gear unit

420

through the motor shaft

414

. The reduction gear unit

420

rotates its output shaft

422

at a reduced speed according to a predetermined reduction ratio, using a mechanism called traction drive, in which shear stress of traction oil is utilized to transmit power.

The motor

410

consists of a cylindrical casing

411

and a stator and other elements accommodated therein. The rear end of the cylindrical casing

411

on the opposite side of the reduction gear unit

420

is closed by a rear cover

412

, and on the rear end side of this cover is attached a fan cover

413

. A rear bearing

415

is fitted in the rear cover

412

, and a front bearing

416

is fitted in a coupling casing

421

that interconnects the motor

410

to the reduction gear unit

420

. The motor shaft

414

(or driving shaft, rotating shaft) is supported at both ends by these bearings

415

,

416

and is coaxial with the center line L of the reduction gear unit

420

.

The casing of the reduction gear unit

420

is made up of three parts: a center casing

423

disposed in an axially central position, the coupling casing

421

mentioned above connecting the reduction gear unit

420

to the motor

410

, and a front casing

424

on the opposite side of the motor

410

. The coupling casing

421

of the reduction gear unit

420

doubles as the casing of part of the motor

410

so that the reduction gear unit

420

and the motor

410

are joined together through this coupling casing

421

. A simple planetary roller mechanism

500

that performs the traction transmission is mounted inside a space defined by the center casing

423

, coupling casing

421

, and front casing

424

.

The simple planetary roller mechanism

500

includes a sun roller

501

serving as a friction (or traction) roller, a plurality of (four in this example) hollow cylindrical planetary rollers

502

in rolling contact with the outer periphery of the sun roller

501

. The inner periphery

503

of the center casing

423

functions as a ring roller, and the planetary rollers

502

are in contact therewith as shown in FIG.

3

.

The ring roller or the inner periphery

503

of the center casing

423

has an inner diameter D

3

slightly smaller than the sum of the diameter D

1

of the sun roller

501

and the double of the diameter D

2

of the planetary rollers

502

, so as to apply pressure between the sun roller

501

and planetary rollers

502

. The ring roller or the center casing

423

is a stationary element of the simple planetary roller mechanism

500

, and so the center casing

423

is fixed to the coupling casing

421

with a through bolt

504

. The sun roller

501

is an input element, and a carrier

505

supporting the planetary rollers

502

is an output element of the mechanism.

The carrier

505

includes four pins

509

each inserted in the planetary rollers

502

, so that the revolving motion of the planetary rollers

502

around the sun roller

501

is transmitted to the carrier

505

. The output shaft

422

of the reduction gear unit

420

, a base part

507

in which is encased a bearing

506

for supporting the distal end of the motor shaft

414

, and a flange

508

protruding outwardly from the base part

507

are all interconnected to each other and they all together function as the carrier

505

. The output shaft

422

is rotatably supported by bearings

510

and

511

inside the front casing

424

.

How the reduction gear unit

420

works will be briefly described below.

Rotation of the motor shaft

414

is transmitted to the sun roller

501

of the simple planetary roller mechanism

500

. The planetary rollers

502

rotate around their axes and revolve around the sun roller along the inner periphery

503

of the center casing

423

, and their revolving motion around the sun roller is transmitted through the carrier

505

and output from the output shaft

422

which is part of the carrier

505

. The reduction ratio x

1

of such simple planetary roller mechanism

500

can be expressed as x

1

=D

1

/(D

1

+D

3

), where D

1

is the diameter of the sun roller

501

and D

3

is the inner diameter of the ring roller (inner periphery

503

of the center casing

423

).

In the power transmission described above in which a chain drive using the chain

305

and sprockets

304

,

403

and the planetary roller-type, motor built-in reduction gear set

401

are combined, there is hardly any noise or vibration caused by resonance between the polygonal motion of chain and the reducer

401

, while stable and reliable torque transmission is achieved.

FIG. 4

is a plot of vibration (G) versus motor rpm showing a result of an experiment in which the system of the present invention is compared to the prior art conveyor drive system shown in

FIGS. 9A and 9B

.

The curve plotted with solid circular marks represents the case with the present invention, while that with solid triangular marks represents the case with the prior art. The graph clearly shows that vibration and resonance are both much reduced in the system of the present invention as compared to the prior art.

The horizontal axis represents rotation per minute (rpm) of the motor accelerated by frequency-converted inverter power supply, and the vertical axis displays vibration acceleration (G) at a point indicated at x in

FIGS. 1 and 9

. In order to make the results more evident, the motor built-in reducer is directly placed on the base without vibration dumper rubber. For the motor, a 3-phase 4-pole motor (0.4 kw output) with a reduction ratio of 1:5 was used. The chain length was the same in both cases, while the diameter of the sprockets in the system of the present invention was made approximately a half of that of the prior art.

The following was observed in the systems used in the experiment:

In the case with the prior art, resonance occurred when the motor rpm was 900 to 1000, 1800 to 2000, and 2400 to 2700 between vibration caused by the “polygonal motion” and that caused by the engagement of toothed gears, resulting in a large vibration at the center of the motor built-in reducer, and noise was accordingly large. In the configuration according to the present invention, despite the use of smaller sprockets, no large vibration occurred over the whole range of the motor rpm.

FIG. 5

is a graph showing a result of another experiment comparing the system of the present invention with the prior art. In this experiment, vibration dumber rubber was provided under the base of the motor built-in reducer and the support legs of the conveyor.

FIG. 5

shows a plot of vibration (G) versus motor rpm similarly to

FIG. 4

, in which the curve plotted with solid circular marks represents the case with the present invention, while that with solid triangular marks represents the case with the prior art. The graph clearly shows that vibration is much reduced over the whole range of the motor rpm in the system of the present invention as compared to the prior art.

According to the present embodiment, it is possible to eliminate resonance over the whole range of motor speed without resorting to increasing the pitch circle diameter of sprockets or the like for use with chains. Thus the above-mentioned driving rotary member of the transmission unit may have a pitch circle diameter greater than that of the driven rotary member by, for example, 1.4 times or more. A desired reduction ratio can be achieved by this difference in the diameters between two rotary members, whereby the motor built-in reducer can be made even smaller.

Furthermore, the simple planetary roller mechanism has a power inputting sun roller, a fixed ring roller, and a power outputting planet carrier. This configuration allows power from the motor shaft to be readily transmitted, and also allows the ring roller to be made integral with a casing, which contributes to the compactness of the mechanism. Furthermore, since planetary rollers transmit power by revolving around the sun roller while rotating around their own axes, vibration from the motor side and the chain side is effectively shut out.

FIG. 6A

is a top plan view and

FIG. 6B

is a front view of a conveyor and a conveyor belt drive system according to a second embodiment of the present invention. In the description of the following various other embodiments, parts or elements identical or similar to those of the first embodiment shown in

FIGS. 1 through 3

are given the same reference numerals, and the description thereof will be omitted. Only the parts different from the first embodiment are given different numerals and will be explained.

The conveyor

600

in this embodiment differs from the conveyor

300

of the previous embodiment in that its driven sprocket

601

is as large as the one in the prior art shown in

FIGS. 9A and 9B

. That is, only the drive sprocket

403

is reduced in diameter, while the driven sprocket

601

is the same size as that of the prior art, whereby speed is reduced, i.e., torque is increased, by the power transmission by the chain

305

. Thereby the motor built-in reducer

401

can be made smaller, lighter, and more compact. The more compact the motor built-in reducer

401

is, the less vibration occurs. Therefore smaller reducers

401

will contribute to a further reduction in vibration in the whole system.

FIG. 7

is a cross section of a conveyor and a conveyor belt drive system according to a third embodiment of the present invention. In this embodiment, instead of the chain

305

for transmitting driving power, a toothed timing belt

700

made of rubber or plastic is employed. In order to install the timing belt

700

, a toothed driven belt pulley

701

and a toothed driving belt pulley

702

are used in place of the driven sprocket

304

and drive sprocket

403

.

FIG. 8

is a cross section of a conveyor and a conveyor belt drive system according to a fourth embodiment of the present invention. This embodiment employs the belt and pulley structure as the previous embodiment shown in FIG.

7

. In addition, two sets of simple planetary roller mechanisms

802

and

803

are equipped in the reduction gear unit

801

constituting the motor built-in reducer

800

, thereby enabling to achieve a wider range of reduction ratio.

As described above, the conveyor and the conveyor belt drive system using a chain or a timing belt and a motor built-in reducer according to the present invention employs a simple planetary roller-type, motor built-in reducer as the power source. This eliminates resonance resulting from a polygonal motion of the chain or timing belt as it runs around the sprockets or pulleys while bending. The noise is thereby much reduced and also the system is made more compact.