GB2324980A - Low noise formation of hollow core concrete slabs - Google Patents

Abstract

Hollow core concrete slabs are extruded through a packing chamber by a plurality of augers 20 with forming elements 36 at their downstream ends and internal vibrators in the forming elements. A top plate 24 of the packing chamber also carries vibrators and is vibrated at a rate between 2500 and 4200 vibrations per minute while the internal forming element vibrators operate at a rate between 6500 and 7600 vibrations per minute. The low vibration rates are found to contribute to a good quality concrete slab, while reducing the sound emissions from the apparatus to well below 85 dB. The top plate vibrations are isolated from the rest of the machine by rubber plate mounts.

Description

LOW NOISE HOLLOW CORE SLAB EXTRUDER The present invention relates to improvements in the formation of hollow core slabs. One aspect of the invention relates to a method of forming an extruded, hollow core concrete slab, said method comprising: delivering fresh concrete to and through a packing chamber with a plurality of augers extending into the packing chamber; forming hollow cores in the concrete with a plurality of forming elements in the packing chamber adjacent downstream ends of the respective augers; vibrating the forming elements; and vibrating a top plate forming a top side of the packing chamber.

Another aspect of the invention relates to an apparatus for forming an extruded hollow core concrete slab, said apparatus comprising: a packing chamber for forming fresh concrete into a slab, the packing chamber including a top plate forming a top side of at least part of the packing chamber; a plurality of augers extending into the packing chamber for delivering fresh concrete to and through the packing chamber; a plurality of forming elements in the packing chamber adjacent downstream ends of respective ones of the augers for forming hollow cores in the concrete slab; forming element vibrating means; and top plate vibrating means.

In the formation of extruded hollow core concrete slabs, concrete may be forced into and through a packing chamber by augers extending into the chamber. Forming elements following the augers form hollow cores in the concrete as it is forced into a slab in the packing chamber. In order to provide a concrete product of the appropriate strength and density. The forming elements and an upper hammering plate, forming the top of the packing chamber are vibrated to plasticize and consolidate the concrete.

A problem with such machines is the very high noise level generated in operation. This may be in the range 105 to 115 dB. This compares with a desired or, in some jurisdictions, mandatory, level of 85 dB. It is conventional understanding that the noise level is due to the vibrations applied to the concrete in forming. Vibration rates of 9000 to 15000 vibrations per minute (vpm) are conventionally considered necessary to plasticize and consolidate the concrete adequately. Reference may be made to U.S. patent 3,224,064 issued 21 December 1965 to R.S. Hummel et al; U.S. patent 3,994,639 issued 30 November 1976 to F. M. Hewitt; and U. S. patent 4,755,338 issued 5 July 1988 to Harala. All of these patents refer to a vibration rate in excess of 9000 vpm.

United States patent 4,710,112 issued 1 December 1987 to Martens describes a forming element vibration rate of at least 22000 vpm.

Attempts to solve the noise problem have included using shear compaction to consolidate the concrete rather than vibrations. This approach is disclosed in United States patent 4,574,064 issued 4 March 1986; United States patent 4,539,165 issued 3 September 1985; United States patent 4,755,338 issued 5 July 1988; and European patent specification 0125825 published 21 November 1984, all of which are assigned to Oy Partek AB. In European patent specification 0174923, published 19 March 1986 of Rakennusvalmiste Oy, a shear action core forming element is described.

United States patent 5,238,374 issued 24 August 1993 to Martens discloses the use of rotary vibrators on a hammering plate operating on axes inclined to the direction of extrusion of the slab. In conjunction with these inclined vibrators, the patent discloses the reduction of the vibration rate to less than 4000 vibrations per minute, resulting in a significant reduction in sound levels.

The present invention is concerned with a method and apparatus with further reduced sound emissions.

According to the method aspect of the present invention, the method is characterized by: vibrating the forming elements at a frequency in the range 6500 to 7600 vibrations per minute; and vibrating the top plate at a frequency in the range 2500 to 4200 vibrations per minute.

The forming elements are preferably vibrated at a frequency in the range 6600 to 7500 vibrations per minute, most preferably substantially 7500 vibrations per minute.

The preferred top plate vibration frequency is substantially 3500 vibrations per minute.

It is also preferred that the vibrations of the top plate are isolated from the rest of the machinery. This is found not only to reduce noise levels but to result in an improved product.

The vibrations of both the core forming element and the top plate are thus significantly reduced. It has been found that this can be done while producing a concrete slab of acceptable or improved density and strength. The manufacturing energy requirements are also significantly reduced.

It is found that when the vibrations of the core forming element are reduced, the vibrations are absorbed in the mass of concrete, while providing adequate plasticity and compaction of the concrete throughout. It has been found that higher vibration rates can be detrimental in contributing to sagging of the concrete over the hollow cores. The vibrations of the top plate are reduced to the level that they contribute predominantly to the finishing of the top of the slab and not to its overall consolidation.

It has been found that with these vibration rates the sound level is reduced well below the industry standard target of 85 dB. Using a forming element vibration rate of 7500 vpm and a top plate vibration of 3800 vpm has resulted in sound levels consistently in the range 77 to 82 dB.

According to the apparatus aspect of the present invention, the apparatus is , characterized in that: the forming element vibrating means vibrate the forming elements at a rate in the range 6500 to 7600 vibrations per minute; and the top plate vibrating means vibrate the top plate at a rate in the range 2500 to 4200 vibrations per minute.

In preferred embodiments, the apparatus uses the "Ribbon"w auger described in United States patent 4,710,112 issued 1 December 1987 to Martens. It may also use the convergent rotary vibrators described in United States patent 5,238,374 issued 24 August 1993 to Martens. The reduced, controlled vibration rate can be achieved by supplying power to independent vibrator motors in the forming elements through a frequency inverter that provides control of the operating speed of the motors. In prior apparatus, such control has not been exercised and motors have been driven at a speed dictated by the frequency of the local mains power supply.

The top plate vibrations may be isolated from the apparatus frame by a top plate suspension means including vibration isolating means preferably comprising a plurality of self damping resilient members suspending the top plate on the top of the packing chamber.

In the accompanying drawings, which illustrate an exemplary embodiment of the present invention: Figure 1 is a side view, partially broken away of an apparatus according to the present invention; Figure 2 is a side view of a portion of the apparatus, partially in section; Figure 3 is a top view of the apparatus; and Figure 4 is an end view, partially broken away, of the apparatus.

Referring to the accompanying drawings there is illustrated a concrete slab extruding apparatus 10 that includes a frame 12 with supporting wheels 14. The wheels are mounted on a pair of rails (not illustrated) in the conventional manner.

The frame carries a hopper 16 which supplies fresh concrete 17 downwardly into an extruding chamber 18 where it is picked up by augers 20 for delivery into a packing chamber 22. Above the packing chamber adjacent the hopper is a hammering or top plate 24. Downstream of the top plate is a trowelling plate 26. The augers 20 are constructed according to the applicant's United States patent 4,710,112, issued 1 December, 1987. Each auger has a core 28 and helical flighting 30. The flighting has a constant diameter from end to end, concentric about the axis of rotation 32. The helical surface 34 of the core 28 spirals outwardly along the auger while remaining parallel to the axis of rotation 32. The downstream end of each auger is a forming element 36 that forms a hollow core in a slab being extruded. Following the forming element is a core finishing follower tube 38. This is coupled to the forming element through a rubber mount 40 to isolate the follower from vibrations of the auger and former 36 as discussed below.

Within each forming element is a vibrator cartridge 42. This includes a rotating vibrator impeller 44. The impeller is driven through coupled drive shafts 46 and 48 extending along the hollow centre of the auger 20 to a vibrator motor 50. The vibrator motor is a synchronous AC motor powered by AC current from a variable frequency inverter 52. The inverter is used to control the speed of the vibrator motors.

The end of each auger opposite the former is connected to an auger drive shaft 54 mounted in a bearing housing 56. The shaft 54 carries a sprocket 58 driven by a chain (not illustrated) from the output of a gearbox 60 The gearbox is in turn driven by a motor 62.

The top plate 24 has a top plate liner 64 mounted on its lower surface. This is a thin, low friction liner plate. On the top of the top plate is a vibrator mounting base 66 carrying two vibrators 68. arranged side by side above the top plate. Each vibrator consists of a motor 70 with an axis of rotation 72 and a series of eccentric weights mounted on the motor drive shaft. The weights are mounted on both ends of the motor shaft and each preferably consists of a series of metal plates that can be added and subtracted to vary the weight driving the vibrations.

In the embodiment illustrated in the drawings, the two vibrator motors 70 are arranged with their axes 72 converging towards the center of the slab in the direction of extrusion. This has been found to produce a lateral flow of concrete ir e packing chamber towards the sides of the slab. Each of the motors 70 is mounted adjustably on the base plate 66 to permit adjustment of the angle between the axis of rotation 72 and the extrusion direction. The maximum range of this angle will normally be 0 to 90 .

The top plate 24 is mounted on a vibration isolating suspension. This includes eight rubber mounts 74 arranged in two rows, along opposite edges of the top plate. The plate is mounted on the bottom ends of the rubber mounts.

The top ends of each row of mounts are connected to a respective one of two transverse suspension beams 76. Each beam is an inverted T shape, with a bottom flange 78 and an upright flange 80. Cutouts 82 in the upright flange 80 accommodate the mounting studs and nuts 84 for the rubber mounts. The ends of the suspension beams are mounted in top plate adjustment frames 86 carried by the extruder frame 12. The adjustment frame carries a vertical bolt 88 passing through a bore in the end of the suspension beam flange 78. Two adjusting nuts 90 vary the vertical position of the suspension beam and thus the compression of the rubber mounts in use. The rubber mounts act as a self damping resilient suspension that isolates and concentrates the top plate movement. They limit vibration transmission to other parts of the extruder, reducing the noise generated. This also reduces the vibration energy required for compaction. The adjustablity of the rubber suspension allows control of the magnitude of the top plate vibrations and results in a product that has improved dimensional accuracy.

It has now been found that a relatively low vibration rate for both the vibrator motors 70 and the forming element vibrators 42 produces a good quality hollow core product while generating considerably less noise than the prior art.

The intemal forming element vibrators may be operated at a vibration rate of from 6500 to 7600 vibrations per minute, with the currently preferred rate being 7500 vibrations per minute. It is found that the vibration energy generated at this rate is all absorbed in the concrete mass surrounding the forming elements and that sufficient energy is transmitted into that concrete mass that it plasticizes and compacts to form a good quality product. It has been found that excessive vibration rates have no benefit and may, in fact, have a negative effect in producing higher sagging over the hollow cores.

The vibrator motors 70 operate at a speed between 2500 and 4200 RPM, delivering the same vibration rate. The currently preferred motor speed is the standard motor speed of 3,600 RPM with 60 Hz AC power. It has been found that this vibration rate on the top plate serves to finish the top of the slab, while the internal vibrators provide satisfactory compaction of the concrete in the packing chamber. In the prior art, it was conventional to vibrate the top plate as a hammering plate at a much higher speed, usually a nominal speed of 10,800 RPM with 60 Hz AC power or 9000 RPM with 50 Hz AC power, in order to provide compaction. The reduced speed of motors 56 contributes to a significant reduction in the sound emitted during operation of the apparatus.

While one embodiment of the present invention has been described in the foregoing, it is to be understood that other embodiments are envisaged and are intended to be included within the scope of the invention. For example, while the illustrated embodiment uses convergent top plate vibrator axes, alternative orientations of the vibrators, including the conventional transverse axis arrangement may be used. In addition, while the invention has proven especially effective when used in conjunction with augers of the type described in United States patent 4,710,112 issued 1 December 1987, the invention is not limited to this use. The forming elements of the disclosed embodiment are part of the augers. In other embodiments, particularly where non-circular cores are to be formed, the formers may be stationary, an arrangement that is described in United States patent 4,968,236, issued 6 November, 1990. The invention is thus to be considered limited solely by the scope of the appended claims.

Claims (15)

1. A method of forming an extruded, hollow core concrete slab, said method comprising: delivering fresh concrete to and through a packing chamber with a plurality of augers extending into the packing chamber; forming hollow cores in the concrete with a plurality of forming elements in the packing chamber at downstream ends of the respective augers; vibrating the forming elements; vibrating a top plate forming a top side of the packing chamber, said method being characterized by: vibrating the forming elements at a frequency in the range 6500 to 7600 vibrations per minute; and vibrating the top plate at a frequency in the range 2500 to 4200 vibrations per minute.

2. A method according to Claim 1 comprising vibrating the forming elements at a frequency in the range 6600 to 7500 vibrations per minute.

3. A method according to Claim 1 comprising vibrating the forming elements at a frequency of substantially 7500 vibrations per minute.

4. A method according to any preceding claim comprising vibrating the top plate at a frequency of substantially 3500 vibrations per minute.

5. A method according to any preceding claim comprising supporting the top plate with a vibration isolating suspension.

6. Apparatus for forming an extruded hollow core concrete slab, said apparatus comprising: a packing chamber for forming fresh concrete into a slab, the packing chamber including a top plate forming a top side of at least part of the packing chamber; a plurality of augers extending into the packing chamber for delivering fresh concrete to and through the packing chamber; a plurality of forming elements in the packing chamber adjacent downstream ends of respective ones of the augers for forming hollow cores in the concrete slab; forming element vibrating means; and top plate vibrating means, characterized in that: the forming element vibrating means vibrate the forming elements at a rate in the range 6500 to 7600 vibrations per minute; and the top plate vibrating means vibrate the top plate at a rate in the range 2500 to 4200 vibrations per minute.

7. A method according to Claim 8 wherein the forming element vibrating means comprise means for vibrating the forming elements at a frequency in the range 6600 to 7500 vibrations per minute.

8. Apparatus according to Claim 6 wherein the forming element vibrating means comprise means for vibrating the forming elements at a frequency of substantially 7500 vibrations per minute.

9. Apparatus according to any one of Claims 6, 7 and 8 wherein the top plate vibrating means comprise means for vibrating the top plate at a frequency of substantially 3500 vibrations per minute.

10. Apparatus according to any one of Claims 6 to 9 including top plate suspension means for supporting the top plate on a frame of the apparatus, the plate suspension means including vibration isolating means for limiting transmission of vibrations from the top plate to the frame.

11. Apparatus according to Claim 10 wherein the vibration isolating means comprise a plurality of self damping resilient members suspending the top plate on the top of the packing chamber.

12. Apparatus according to Claim 11 wherein the top plate vibrating means comprise rotary vibrators mounted on the top plate.

13. Apparatus according to any one of Claims 6 through 12 wherein each auger has an axis of rotation, a core with a helical, spiraled surface substantially parallel to the axis of rotation and helical auger flighting of substantially constant diameter.

14. Apparatus according to Claim 13 wherein the top plate vibrating means comprise rotary vibrators mounted on the top plate, rotating on axes that not perpendicular to the axes of rotation of the augers.

15. Apparatus according to any one of Claims 6 through 14 wherein the forming element vibrating means comprise electric motors and frequency inverter means for controlling the speed of rotation of the motors.