Re: Rockbox V Impact Plate Transfers
Very interesting David.. One point though:
Many people believe that the material in the dead box slides off the dead material at the angle of repose, which is not really the case.
What actually happens, especialy with higher belt speeds, is that a semi-dead cone of material forms in the trajectory into the dead box. This cone points towards the material leaving the discharge pulley, and grows when the material is wet and sticky and retracts when it is more free flowing.
Because of this, you have to allow enough room between the dischargs pulley and the dead box to accommodate the worst case length of this cone to avoid blockages.
The material flowing beneath the cone maintains a good velocity, and the material flowing round the sides slows down. It can be seen that the material flowing on top of the cone can stop dead.
I try to use this phenomenon in my four part adjustable dead box arrangement to direct the different velocity streams to the correct parts of the chute below to give central loading.
Incidentally, I have now developed my own computer model for flow in chutes from first principles. The results from a model of a huge chute that I did recently were audited by TUNRA. They got the same answers as me, but interestingly enough another company did (an expensive) check using a seperate computer model using discrete element analysis, the results of which were a joke!
Cheers
LSL Tekpro ■
Re: Rockbox V Impact Plate Transfers
CHUTE MODELLING
Thanks for those comments Graham. With regard to chute modelling; I have been asked on many occasions to review and comment on chute designs created by others and I am often amazed that generally there has been no attempt by the designer to show where they think the material flow will go! An attempt at determining the approximate flow path from first principles should always be carried out and this can usually be developed allowing for a range of possible velocity/discharge slope scenarios. For many years we kept a bucket of vermiculite granules in the office, which we poured through scale cardboard models of a chute to check our predictions. These methods may be basic in today’s computer driven world but we had a high success rate and a reputation for getting it right first time. These simple methods are a hell of a lot better than totally ignoring the material flow and hoping it will work out ok when it is built.
Interestingly there is a website that I was looking at the other day for a company offering a DEM chute design service and the site includes a working example of a chute design. In this example the material flow in the lower part of the chute clearly has lateral bias and if the chute had been built as shown it would, I believe, have caused belt tracking problems. I think the message here is that the best computer model in the world is no use unless the person using it knows what they are trying to achieve and why.
Your four part rockbox sounds interesting; do you have a typical illustration that you could send me?
Regards
Dave Beckley
david.beckley@cdcwa.com.au ■
Chute Modelling
Dear Mr. Beckley,
thank you for putting up this interesting thread.
I would like to add two points.
First, there is sometimes the basic configuration of a moving discharge chute, either linear (1 line discharge, 2 lines to feed) or rotating (type stacking / reclaiming devices).
There the side winged impact plate proved to be a good choice for me, even if the result was not always 100% in terms of lateral bias for the receiving belt.
This is mainly due to the geometrical restrictions at the receiving chute.
These problems were taken care of by the design of the impact idler stations (and the receiving chute) below the impact zone. It is necessary to consider the length of the acceleration zone to avoid spillage / belt deflection in the area of garland type idler stations under different conditions.
Secondly, there are situations when construction space is restricted within a transfer point building, or when throughput is increased well above the level designed initially. Then the problems arising from the distance impact plate - discharge pulley (primary diversion) become significant. There will be the building-up of an impact body, which sometimes may lead to the blocking of the passage. Here the maintaining of sufficient flow velocity in the outer areas of the flow body is of great importance. In this case we use the modelling of the materials flow based on the continuous flow method in order to play through several possible scenaria of construction geometry, speed of belt, characteristics of material etc. This gave sufficiently good results, proven by implementation.
Also there's now the possibility to define a adapted control algorithm in order to change the angle of the impact plate by a linear drive or other device.
Regards,
Roland Heilmann ■
Wear Back Curved Chutes
Lately they have combined the curved chute design with the rock box principle to allow for high flow rates with low wear rates.
See attached file for some information
Best Regards,
Gareth Blakey
Attachments
wearback design principles (PDF)
■
Re: Rockbox V Impact Plate Transfers
Thanks for that Gareth.. interesting.
I have a problematic coal chute at present. The dead box packs up with fine wet clayey coal.
I though of using a curved baffle on top to direct the coal downwards. But I have just realised though, that you can't use a baffle when you have an overband magnet!
So.. I am going to slow the belt down a bit.
Cheers
LSL Tekpro ■
Re: Rockbox V Impact Plate Transfers
Incidentally Gareth..
We do have these sort of chutes here in South Africa. They are not called WEar BAck chutes here, but they are WEBA chutes.
This is bacause they are from a chap called WErner BAller, who I think probably thought of the principle in the first place
Cheers
LSL Tekpro ■
Re: Rockbox V Impact Plate Transfers
Graham,
Yes I am aware of this.
His chutes are also used in Australia.
However apprently he could not get his product patented in Australia so others are now copying the principle. ■
Re: Rockbox V Impact Plate Transfers
Gareth,
Thanks for posting the info on the WAMS chute. These chutes could be a good topic for the Society.
This design still incorporates a sloping rockbox arrangement at the top that should work satisfactorily on the inline transfer shown but with an angle transfer it is most likely that this upper rockbox will still induce some lateral bias. You know my views on the importance of a vertical stream from the primary diversion in relation to angle transfers. Have you observed the material flow from an angle transfer of this type?
Regards,
Dave Beckley. ■
Re: Rockbox V Impact Plate Transfers
Dave,
Yes, They can achieve perfect flow to the point that skirts are not needed.
The shelves are adjustable and are tweaked during commisioning.
They have already agreed to do a presentation in June. ■
ROCKBOX v IMPACT PLATE TRANSFERS
IMPACT PLATES v ROCK BOXES
Following from recent dialogue between Joe Dos Santos, Graham Spriggs, Larry Nordell and myself in the thread on ‘Conveyor Belt Tracking’ in this forum; as this tread was starting to get away from the original topic, I decided that a separate thread on Impact Boxes v Impact Plates might be of interest to users of this forum. At the risk of being censored for being too verbose, I will firstly cover the basics and then invite comment on this topic.
For the benefit of those who have not experienced these terms and their uses, it is common practice in belt conveyors transfer chutes to intercept the material discharge trajectory at the head end of the conveyor to divert the flow downwards where it can then be diverted in the direction of belt travel of the outgoing conveyor. These two changes in the material flow path being generally known as the primary diversion and the secondary diversion. These diversions can be achieved by using rock boxes, flat impact plates or curved chutes or a combination of two of these three methods. The objective in each case is to load the material centrally without lateral bias, in the direction of belt travel and with a horizontal velocity component that is approximately equal to the belt velocity.
A rock box usually comprises a shelf with a wear lip along the discharge side or sides. The conveyed material flows onto the rock box, where it builds up to a specific angle, that is associated with the material properties, and then flows off the rock box roughly at the angle of the outer face of static material in the box. In many cases the position of the discharge lip is made adjustable to change the flow position. Rock boxes can be used for both the primary and secondary diversions. The main advantage of rock boxes is their relative simplicity and the fact that most of the wear is material on material. The main disadvantage is that due to the fact that the flow from the primary rockbox will have a horizontal velocity component that can cause belt tracking problems.
An impact plate is usually flat when viewed from the side but when viewed from above it should have side wings that are bent forwards at the outer edges at 30 to 35 degrees so that when viewed from above it has a shape similar to that of the idler trough. These wings are usually about a third of the width of the plate and are necessary to concentrate the material flow in the centre of the chute. These impact plates can be protected by using very heavy duty rubber components such as those manufactured by Trellaborg or by using micro rock ledges on the face of the plate. The plates must be set up to give a vertical material stream. Impact plates can be used in conjunction with rock boxes, inclined plates or curved chutes for the secondary diversion.
Curved chutes often called ‘hood and spoon transfers’ can work extremely well but in hard rock mining conveyors they can suffer from high wear problems. The material flow from the top hood should also be vertical.
In the 1960’s, in the then fledgling iron ore industry in Western Australia, rock boxes were the standard design. Around 1969 Hamersley Iron started experimenting with impact plates and as they found that the impact plates gave better material flow control in their high capacity transfers and less belt tracking problems, they became the flavour of the month and continued to serve the industry well for many years. Many angle transfers that were originally fitted with rock boxes where changed to the impact plate design. More recently, as throughput rates and belt speeds have increased, the impact plates reached their flow limit and today more and more curved hoods are being used. In the Australian coal industry hood and spoon transfers are the norm.
While I have designed 90 degree rockbox transfers that have worked well, their success was dependent on the use of an ingeniously angled discharge lip on the secondary rockbox that directed some of the flow to one side to counteract the biased lateral flow from the primary rockbox and this required site adjustment to avoid belt drift issues. For most applications I have a strong preference for getting the material flow after the primary diversion vertical, hence my recommended use of impact plates of one form or another.
I would be interested to know what experiences other people, particularly from hard rock mining operations, have had with these three different types of chutes.
Regards,
David Beckley
Conveyor Design Consultants of WA
Perth, Western Australia.
www.cdcwa.com.au ■