US 3881662 A
A quite brush chipper having a bladed cutter head and a flywheel has the moment of the flywheel increased so that the combined kinetic energy of the cutter head and the flywheel at 1,550 RPM is at least equal to the combined kinetic energy of these elements in prior chippers at 3,000 RPM, so that the quiet chipper can be driven at 1,200 - 1,600 RPM for reducing the noise level while providing sufficient stored kinetic energy for effective intermittent chipping.
Description (OCR text may contain errors)
United States Patent 1 Gunnarsson May 6,1975
[ REDUCED NOISE LEVEL BRUSH CHIPPER  Inventor: Arne N. Gunnarsson, Pomona,
 Assignee: FMC Corporation, San Jose, Calif.
 Filed: Nov. 21, 1973  Appl. No.: 418,006
 US. Cl 241/221; 241/292.1  Int. Cl. B02c l/08  Field of Search 241/221, 222, 220, 198 R,
241/189 R, 186 R, 186.2, 285 R, 291, 292.1
 References Cited UNITED STATES PATENTS 2,345,779 4/1944 Wagner 241/189 X 2,679,873 6/1954 Hill 241/186 X 3,270,968 9/1966 Hess et al 241/221 3,771,733 ll/1973 Hadley et a1 241/186 R Primary Examin'erRoy Lake Assistant Examiner-DeWalden W. Jones Attorney, Agent, or FirmC. E. Tripp  ABSTRACT A quite brush chipper having a bladed cutter head and a flywheel has the moment of the flywheel increased so that the combined kinetic energy of the cutter head and the flywheel at 1,550 RPM is at least equal to the combined kinetic energy of these elements in prior chippers at 3,000 RPM, so that the quiet chipper can be driven at 1,200 1,600 RPM for reducing the noise level while providing sufficient stored kinetic energy for effective intermittent chipping.
2 Claims, 5 Drawing Figures PATENTEE HAY 5 I975 SHEET 10F 3 PATENTEBHAY 61975 SHEET 2 BF 3 T'IE'I IEI REDUCED NOISE LEVEL BRUSH (IHIPPER FIELD OF THE INVENTION This invention relates to wood chippers for branches, brush, etc. and more particularly to the reduction in the noise level of these devices.
DESCRIPTION OF PRlOR ART A brush chipper cutter head of the type to which this invention relates is exemplified by the US. Pat. No. to Hall 3,195,592, July 20, 1965. Such brush chippers include a housing which mounts a rotary, generally cylindrical cutter head having a plurality of cutter blades that run very close to a fixed bed knife. Although not shown in the Hall patent, these chippers customarily include a chip disposal chute and the cutter head shaft mounts a pulley that is driven by a source of power, usually a gasoline engine. The other side of the cutter head shaft mounts a flywheel to increase the kinetic energy of the rotating assembly and usually includes a blower which directs air to a disposal chute for assistance in moving chips out the chute and into a receptacle.
When properly adjusted, the sharpened blades on the cutting head or rotor just clear the bed knife, the clearance being a maximum of about one-sixteenth of an inch. Typical prior chippers of this type, having a cutting head of about 1 l to 13 inches in diameter, must be operated at a minimum speed of 2,200 rpm and often up to a speed as high as 3,000 rpm. These speeds are required with chippers of this type in order to provide adequate chipping for medium or heavy branches without stalling the power source, usually an internal combustion engine. The engine is fitted with an automatic governor which is set to maintain the engine at a nominal operating speed, both when the machine is at standby in between chipping operation and during the actual chipping operation, although the engine does slow down somewhat under the chipping load. This governed speed, which may be as high as 3,000 RPM (for example) will be referred to herein as stand-by speed or operation.
Experience shows that chippers of this type run at standby, that is when they are not being used for cutting or chipping, for about 88 to 95 percent of the total running time of the machine. During this idling time, the cutter knives, which run close to the bed knife, act as a siren and create an objectionable noise. Such noise has a level high enough that prolonged exposure to the noise at distances in the order of feet or so, can damage the human ear. it is also known that the damage is accumulated, meaning that an operator who is exposed to the noise over a substantial period of time can suffer permanent auditory damage, as compared with minor and usually transitory damage sustained by those who are only subjected to the noise occasionally. For example, prior chippers of the type described may, when at stand-by, have a noise level at about 20 feet in the front or rear of the machine that is equal to or exceeds 100 dB measured on the A scale by a standard, commercial type noise level meter. This noise level, which is within the range of noise produces by a pneumatic peening hammer, not only occurs during the relatively small fraction of the time during which the machine is chipping but is generated during the major portion of the running time of the machine, and has been considered objectionable psychologically to those close to the machine. Also, this noise level is unacceptable under present government regulations, such as the Noise Level Standards of the Federal Office of Safety and Health Administration (OSHA).
The chipper of the present invention operates at a reduced noise level without impairing chipping efficiency. Previous chippers, operating at an objectionably high noise level, have been designed to rotate the cutter head at speeds ranging from a minimum of 2,200 rpm to 3,000 rpm. Chippers operating at these speeds produce the objectionably high noise levels previously described. it was conceived that operation of a chipper at lower speeds might reduce the noise level. Experiments showed this to be the case. For example, when running a chipper at about 1,550 rpm, the noise level at stand'by is reduced from that of prior conventional or standard chippers operating at a stand-by speed of 3,000 rpm by about 9 10 dB on scale A of the noise level meter. Specifically, the noise level of the slower machine at the noisiest position, namely about 20 feet from the rear of the machine, is reduced from over 100 dB-A to about dB-A, a decrease of 9 10 dB.
it was found that it is not possible to operate the prior chipper at speeds low enough to reduce the noise to a satisfactory level, because the engine would stall under load and the kinetic energy imparted to the chips by the action of the cutting head was not high enough to satisfactorily throw the chips out of the disposal chute and up into the receptacle or truck bed. Also, the standard machine, when operated at reduced speed, would not satisfactorily be self feeding on brush or the like during cutting. in order to permit operation of the chipper at a stand-by speed low enough to provide satisfactorily low noise levels without sacrificing actual cutting effectiveness, chip disposal and self feeding, under the present invention, it was found that if the mass of the rotating parts were increased so that the kinetic energy of the rotating mass at the lower speed stand-by operation equaled or exceeded the kinetic energy of the rotating mass at the prior high speeds, then the advantages in noise reduction provided by a lower standby speed could be attained without sacrificing cutting and chip disposal effectiveness. More specifically, it was found that the cutting head could remain the same as before but that increasing the moment of the flywheel by a factor of more than two would result in satisfactory overall operation. With this concept, the stored kinetic energy of the cutting head and flywheel asssembly of the improved, quieter machine at an operational speed of about l,550 rpm (for example) somewhat exceeded the stored kinetic energy in the prior machine operated at 3,000 rpm (for example). This modification reduced the noise level and provided adequate performance. It was further found that with improved design of the present invention the frequency or pitch of the noise generated by the machine at lower speeds was reduced by one octave. This reduction in frequency of the stand-by noise of the chipper augmented the effects of reduction in the basic noise level because it has been found that psychologically, the noise generated at a lower frequency, characteristic of the chipper of the present invention, is less annoying than that generated at the higher frequency, characteristic of prior chippers of the type described.
Thus, as a result of the present invention, only is the stand-by sound level of the chipper reduced without sacrificing the effectiveness of operation during cutting, but the frequency of the sound level idling is lowered and these factors decrease the psychological noise effects of the machine.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation of a brush chipper embodying the present invention, with parts broken away.
FIG. 2 is a section taken on line 2 2 of FIG. 1.
FIG. 3 is a plan view of the brush chipper with the parts broken away.
FIG. 4 is an enlarged fragmentary section showing a cutting head and bed knife assembly.
FIG. 5 shows noise level comparison curves between a standard chipper and the quiet chipper of the present invention operating at stand-by.
GENERAL DESCRIPTION OF THE Cl-IIPPER Referring to FIGS. 1, 2 and 3, the chipper of the present invention includes a stand that supports a cutter head housing 12. A cutter head or rotor R is rotatably supported in the housing 12 and carries four blades 14 which run close to a fixed bed knife 16 for chipping. A feed chute 17 projects rearwardly from the cutter housing 12 and a discharge chute 18 projects forwardly from the housing for conveying chips removed by the cutter head from logs or branches L to an associated truck or container. The mechanism just described is mounted on a base or frame partially indicated at 19, it being conventional in this art to construct the frame as a wheeled chassis which not only supports the chipper under description but supports an internal combustion engine or other prime mover for driving the cutter head rotor R.
Rotor Construction A typical cutter head R comprises a drum-like rotor indicated generally at 30 (FIG. 4) and which is provided with four inclined slots 32 for receiving the cutter blades 14. Each cutter blade is formed with a serrated surface 34 which matches a complementary surface formed on a wall of the associated slot 32. Each blade is clamped by a clamp plate 36 and a clamp bolt 38 which passes through a slot (not shown) in each blade 14 to accommodate adjustment for blade wear. As best seen in FIG. 4, the bed knife 16, which is shown as being of hexagonal construction, is clamped in a groove formed by the edge of a wall of the cutter housing 12 and the edge of a frame element 40 by means of one or more clamp bar 42 and a clamp bolt 44. It is to be understood that the details of the cutter blade mounting and of the bed knife construction are not criticai to the present invention, but in all cases the blades 14 run very close to the cutting edge of the bed knife 16 and hence provide a narrow air escape gap indicated generally at g in FIG. 4. This gap, which is one-sixteenth of an inch wide or less, causes the cutter head R to act as a siren, particularly during the 88 95% operating time that represents stand-by operation. Reduction in the level of the noise produced by this siren effect represents a principal consideration of the present invention. Cutter Head Mounting and Fly Wheel The cutter head or rotor R is mounted on a sturdy shaft 50 and the shaft is supported by the housing 12 in a manner of which is not critical to the present invention. As seen in FIG. 2, one end portion of the cutter head shaft 50 is mounted in a side plate 52 of the cutter housing 12 by means of a combined radial and thrust anti-friction bearing 54 mounted in a sieeve 56 projecting from the side plate 52 and located by spacers. The other end portion of the cutter head shaft 50 is mounted in the opposite side plate 58 of the cutter head housing in a sleeve 60 that also supports the shaft in an anti-friction bearing. It is understood that the details of the bearing mount for the cutter shaft 50 are not critical to the invention and suitable constructions are well known in the art.
Referring to FIG. 2, an inner flywheel housing section 62 is secured as by screws (not shown) to the free end of the side plate sleeve 56, previously described. A large flywheel 64 has a hub 66 that is formed with a key way and the hub fits over the reduced diameter, projecting end 68 of the shaft 50, which end also mounts a flywheel key 70. The flywheel 64 and the impeller 72 of a blower are secured to the end portion 68 of the shaft by a bolt and clamp washer assembly illustrated generally at 74, it being understood that the details of the manner of securing the flywheel and blower impeller to the shaft are not critical to this invention and are well known in the chipper art. The blower housing 20, previously mentioned, is detachably secured to the flywheel housing section 62 by means of a flange 76.
In order to assist in ejecting chips from the cutting area of the delivery chute the centrifugal blower impeller 72 has radial impeller vanes 82 and a central inlet throat 84. The blower housing 20, previously mentioned, has an air inlet tube 86 that makes a running fit with the inlet throat 84 of the blower by means of an annular flexible ring 87. An air filter 88 covers the air opening provided by the inlet tube 86. Some installations incorporate a brake assembly, illustrated generally at 90, and associated with the flywheel 64 for quickly. bringing the unit to a stop when necessary. The details of the brake assembly (if present) are not critical to the present invention.
In order to drive the cutter head shaft 50 from an internal combustion engine, or other prime mover, a mul tiple V-belt pulley 92 is connected to and keyed to the end of the shaft on the right side of the machine. The pulley 92 is driven by V-belts from an engine pulley not shown in the drawings, it being understood that this means of driving chipper cutter heads is known in the art.
Ouieting the Chipper Once it had been ascertained that the stand-by noise level of the chipper could be reduced to an acceptable standard by lowering the speed of the chipper cutter head to a minimum of 1,200 RPM (below which chip ejection becomes a problem) or to a maximum of l,550 to 1,600 RPM (above which the noise level is not sufficiently reduced), it became necessary to devise a means for providing effective cutting by the chipper over this lower speed range. Of course, the obvious expedient would be simply to change the ratio of the engine and chipper drive pulleys to cause the chipper to operate at a lower speed. However, this expedient is inoperative because it was found that during actual cutting, the chipper speed would drop below the acceptable lower limits for effective cutting and chip removal. Consideration of the problem indicated the possibility that if the kinetic energy of the rotating portions of the chipper, when operating at speeds such as 1,550 RPM for example, could be maintained equal to or somewhat greater than the kinetic energy of a standard chipper when running at speeds such as 3,000 RPM for example, the stored kinetic energy of the rotating partsof slower speed chipper might be sufficient to perform the intermittent type of chipping operation usually carried out by these machines, and without causing the speed of the cutter head to drop below an acceptable minimum speed of 1,200 RPM.
To give a specific example, a typical cutter head for one of these chippers will be about 1 1 inches in diameter and 12 inches long, with a weight of about 470' pounds. The fly wheel of the above exemplary machine is about 21 inches in diameter, about 2.5 inches wide and weighs about 210 pounds. The kinetic energy of the rotating mass including the flywheel and cutter head of this typical prior assembly is about 198,000 ft. lbs. at 3,000 RPM. In a machine embodying the present invention, and using the cutter head described above, it was decided to increase the kinetic energy of the rotating mass by increasing the moment of the flywheel. This was accomplished by leaving the flywheel width at 2.5 inches, and increasing the flywheel diameter from 21 inches to about 32 inches, giving a flywheel weight of about 532 pounds. As mentioned, the cutter head weight remains at 470 pounds. This gives a kinetic energy of 211,000 ft. lbs. at an RPM of 1,550, which speed, as previously mentioned, produces an acceptably low noise level.
Tests of the quieter machine, having a stored kinetic energy of 211,000 ft. lbs. at 1,550 RPM, showed that during the intermittent cutting operation to which these machines are normally subjected, the speed of the cutter head did not drop below an acceptable value for effective cutting and chip disposal.
FIG. 5 is a composite of four noise level curves A, B, C and D giving the comparison of noise levels of what is termed the standard chipper running at 3,000 RPM, the specifications of which have been mentioned briefly, as compared with the noise level of what has been termed a quiet chipper running at 1,550 RPM. Measurements were taken using a General Radio Sound Level Meter, type 1,565B calibrated by a General Radio Sound Level Calibrator, type 1,562, manu factured by the General Radio Company of West Concord, Mass, U.S.A., the principles of which are described in the Handbook of Noise Measurement, 6th Edition, by Peterson and Gross, published by the General Radio Company.
Test readings were taken on scale A of the meter which scale compensates for the psychological effect of pitch or frequency of noise on the human ear. The readings were initially plotted on a polar chart (not shown) that is, they were taken at various positions circumferentially around the machine and at three distances, namely a 7 meter radius, a 50 foot radius and a 100 foot radius. The test curves of FIG. 5 are selected from the aforesaid set of circumferential readings of both chippers in that they represent readings taken at the left and right hand sides of the chipper (curves A and B) and readings taken at the rear and front of the chipper, (curves C and D).
Examination of the test curves of FIG. 5 show that the quiet chipper of the present invention (solid lines) had a noise level at stand-by of about 9 dB-A less than that of the standard chipper at stand-by (dotted lines) throughout substantially the entire test. A study of the test figures obtained from which the curves of FIG. 5 were plotted shows that the noise level drops off sharply as the cutter rpm is reduced and remains relatively flat in the rpm range of 1,200 to 1,600, which range is selected as the operating range of the quiet chipper of the present invention during cutting.
Further examination of the four curves of FIG. 5 shows that the highest noise levels are at the rear of the machine, curve C. At about 20 feet from the chipper, the noise level of the standard chipper (dotted line) was about 102 dB-A and the noise level at 40 feet about 97 dB-A.
The noise level of the quiet chipper (solid line curve C) at about 20 feet from the rear of the machine, dropped from 102 dB-A for the standard chipper to 93 dB-A for the quiet chipper. Since a noise level drop of 3 dB-A represents halving the power level of the noise, the aforesaid total drop of about 9 dB provided by the quiet chipper decreases the power level of the noise of the quiet chipper to one-eighth of that of the standard chipper.
Curve C also shows that at a distance of about 40 feet, the noise level of the standard chipper was 97 dB-A whereas that of the quiet chipper dropped to 87 dB-A. Curves A,B and D of FIG. 5 show correlated results, but all of these noise levels are somewhat less than those of curve C.
As mentioned, the aforesaid brief description of curve C of FIG. 5 have been directed to conditions at the very loudest zones of operation of the chipper. In order to give some comparison of the intensity level of the noise generated by these chipper operations with the sounds generated under more familiar conditions, the aforesaid Handbook of Hand Noise Measurement, page 5 and the Elements of Physics by Shortley and Williams, Prentice and Hall, Inglewood, N..l., U.S.A. (second edition) page 407 give the following examples of the intensity levels of commonly experienced sounds.
Thus under the present invention, not only has the noise level of the quiet chipper been substantially reduced but since its speed is slower, the frequency of the noise is lower than that from the standard chipper.
Sound level studies have shown that the apparent loudness that we attribute to a sound varies, not only with the sound pressure or intensity but also with the frequency or pitch of the sound. sychologically, it is known that sounds or noises at relatively low frequencies are not as noticeable or objectionable as noise phenomena as are sounds of the same intensity but having higher frequencies. In the quiet chipper of the present invention, with a 4 blade cutter head construction described, the frequency of the idling noise (stand-by) generated by standard chipper running at 3,000 rpm would be about 200 Hz, whereas that of the quiet chipper running between 1,200 and 1,600 rpm will be about to 100 Hz. Psychologically, the apparent loudness of noises of equal sound pressures but at different frequency ranges will not be the same. The apparent loudness of the lower frequency noise will be less than that of the higher frequency noise. This effect, which is a characteristic of the chipper of the present invention, represent another aspect of the reduction in the objectionable characteristics of the noise emanating from the chipper, particularly during stand-by operation, which represents about 80 95% of the total operating time.
In the specific example described in detail herein, the cutter head was 11 inches in diameter and 12 inches long. Actually commercial machines of this type may have cutter heads of l l 16 inches diameter and 12 16 inches length. However, the principles of applying the teachings of the present invention to machines employing cutter heads of different sizes remain the same. The moment of the flywheel will be increased to maintain cutting effectiveness and adequate chip disposal when operating the machine at speeds in the range of about 1,200 1,600 rpm.
Although the best mode contemplated for carrying out the present invention has been herein shown and described, it will be apparent that modification and variation may be made without departing from what is regarded to be the subject matter of the invention.
What I claim is:
1. In a brush chipper of the type comprising a housing having a bed knife therein, a shaft driven cutter head having a diameter of about 1 l 13 inches and a length of about 12 inches, said head mounting cutter blades that project only a short distance from the head, feed and disposal chutes on said housing, means for driving said cutter head; and a flywheel on said cutter head shaft; the improvement wherein said cutter head and said flywheel have a combined kinetic energy of about 200,000 ft. lb. at a rotational speed of about 1,550 rpm,
said means for driving said cutter head shaft driving the I shaft at a standby speed within the range of about 1,200 1,600 rpm for causing the noise level generated at the rear of the chipper as the cutter head blades pass the bed knife to be within a range of about dB-A at feet to about 93 dB-A at 20 feet while providing sufficient stored kinetic energy for effective intermittent chipping.
2. The brush chipper of claim 1, wherein said flywheel is a flat steel disc about 2.5 inches thick and about 30 to 32 inches in diameter.