You first need to determine the force required to overcome resistance on the top surface of the slide gate due to the head of material pressing down on the slide. This formula is simply the gate area times the material overpressure times the coefficient of static friction of the material on the plate. Then add the slidng resistance on the underside according to the method of support by multiplying the first result by the bearing resistance, i.e. if plane bearing or rollers. An allowance must also be made for potential wedging or trapping in the top clearance gap as the slide withdraws according to the composition of the ore. Convert these figures to the tooth load on the rack and, having chosen a suitable pinion size and operating speed for the slide, you have the torque and output speed of the drive shaft. If you are not able to determine these values yourself, you should go to someone who can, perhaps a slide manufacturer as it may well be cheaper than a consultant. ■

In addition to the comments made by Lyn, your calculations may need to make an allowance for closing the slide gate into a static column of material.

I am not aware of a publication that details a formula into which you just have to drop a few numbers. There are companies who specialise in large heavy duty slide gates. Maybe you should just buy one! ■

Designer is right. particularly if the column is static. A slide gate is usually moved through a stream of flowing material where there is some inertia and an increasing load as the exposed area increases. There are also other effects, such as time consolidation and surface cohesion, possible build up on the slide surface and meeting a lump as it tries to close. Probably the most awkward factor to determine is the overpressure acting on the slide as this depends on the hopper shape, impact loading and first fill conditions. I would think that this is beyond the amateur designer who would have to make a crude assessment or base it on valves of which has empirical experience. In the end it come down to the fact stated a few times on this forum; that it is help on the basics of the technology and a guide where to go, but it cannot provide a free design service. ■

A slide gate to handle 150mm lumps of magnetite or hematite falling from 3m is daunting. I can provide a low wear gate design which will save you a lot of investment in gearboxes & the like. No sliding involved.

As Lyn & Designer point out, the closing mechansim is quite complex. If you have to consider forcing the gate into a static column of material then why try to advance the gate any more? Flow has stopped; end of story? But no; because we have to have the gate for the other opening sizes. Would you work under a stopped flow when you could see burden above? Of course not.

So you have a trapped flow & must continue closing. Within reason your trapped lump, 150mm must be lifted about 75mm for the plate to progress underneath. This stone must lift the burden above it before it can itself move. (Unless the knife edge is able to shear through magnetite!) For the full plate to progress it must be able to lift a row, along the leading edge, of rocks 75mm which involves raising the prism of overhead burden. As that burden is lifted the rocks behind might get loosened so the closing force requirement reduces temporarily.

Its a very simplistic approach made in the hope of generating further more learned comment.

Does this approach apply to run of mine situations involving rocks of 1500mm? Maybe, if anybody scaled up the outlet to suit.

If you trap a rock against the far wall then you won't be able to lift it because there is no yield behind it. Then you can get trickling pebbles influencing the attitude of your relining crew. So you have to provide a blind pocket to allow full closure. It must be possible to evacuate the pocket ready for the next closure. I could go on...if I'm not already. ■

John's reply draws attention to other forms of outlet control that avoids some of the potential drawbacks of slide gates. However, a meter square opening is going to impose serious loads on whatever is used to cut off the supply, so is not a task to be undertaken lightly. I would be interested to exchange ideas with you John, as we appear to be situated fairly close together, if you care to contact via lyn@ajax.co.uk ■

Dear Sir,

Designer has pointed out the vital point, but apart from this you have to consider the force required to shear the material against the column load while closing.

Regards.

A.Banerjee ■

What s the purpose of gate?

Is it mounted under a hopper and over a feeder. In that case slide gate is operated only when feeder requires maintenance and material must not flow to the feeder.

If si what is size of the hopper.

It is customary to provide such gates over the feeders feeding material from stockpile to a tunnel conveyor. In this case also gate is normally open and is shut when feeder is to be isolated.

Needle gate is a better alternative for these conditions. ■

Originally Posted by Vinayak

Needle gate is a better alternative for these conditions.

But I have yet to see a needle valve, AKA a rod gate, successfully motorised and motorised gates are what most people want. ■

Originally Posted by designer

But I have yet to see a needle valve, AKA a rod gate, successfully motorised and motorised gates are what most people want.

I saw a design for a pneumatically driven rod gate in that bastion of mediocrity known as St Georges Terrace. It had more chance of catching sharks than providing reliable shut off. How many rods do you need to stop smaller lumps from testing hard hats & shoulder pads?? If the bars are close enough together then how can you fit big enough actuators in the proximity? According to Sir Isaac Newton, what can fall down will fall down. ■

The rack and pinion gate or needle gate for isolating feeder from the material in hopper is required to be operated only when feeder is taken for maintenance. That is a rare occurrence. Also at that man manpower is there near the feeder to troubleshoot. In 1977 we had fitted a needle gate under a 2000 T iron ore silo and over an apron feeder. It worked fine. Only a few needles were out for the required feed rate. and they could be closed manually by one person. There are two layers of needles so you really open or close a small gap. I have used these gates replacing shut of plate with rack and pinion mechanisms in many locations with success. One was under a clinker silo with clamshell gates for truck loading. Once set frequent changes were never required. May be I work in a country where it is still customary and possible to keep an attendant on watch at similar locations. ■

Originally Posted by Vinayak

May be I work in a country where it is still customary and possible to keep an attendant on watch at similar locations.

Lucky you!

I was once 'persuaded' by a whining salesman and his boss to quote for an EP operated rod gate. Unfortunately, we got the order and as I had warned we were never able to get it to work reliably. Still, never let technical considerations get in the way of making a sale.

A word of warning, while they frequently work happily without all the rods removed, this can affect the flow pattern in the bin/silo detrimentally. ■

Originally Posted by Vinayak

...... Only a few needles were out for the required feed rate. and they could be closed manually by one person. ......

A few out of how many.?..because the trained observer will suspect that the outlet was too big in the first place. How fast is the closure?

The original question concerned the power required to close off a gateway & I suspect the starter was thinking more in terms of horsepower, to go back in time, rather than people power. ■

With due respect to others, I had never been comfortable with Rod gates. In many places, I removed the rod gates. As usual opinions differ.

Rgds, ■

IMHO, rod gates can be a cheap and simple cut off device for occasional use.

Clearly, as with most things, they can also be misapplied and poorly designed by those with a limited understanding of their characteristics. ■

Mr.Designer,

Request you to clarify IMHO

Rgds, ■

Originally Posted by sganesh

Mr.Designer,

Request you to clarify IMHO

Rgds,

Ah,

IMO, In My Opinion

IMHO, In My Humble Opinion ■