By Tom Lechtenberg
The bucket elevator continues to be the leading cause of grain dust explosions, with most of the explosions attributable to legs. It is critical to make the bucket elevator as safe as possible.
In retrofitting existing legs to provide more capacity, there are many areas to consider in order to keep your bucket elevator running safely end efficiently. Neglecting any of these can result in a breakdown or problem, which could develop into an explosion.
First determine the required legging capacity and remove bottlenecks. There are normally one or more of the following issues:
• Spouting — Spouts are not large enough to take the grain away from the leg or get grain to the leg.
• Horsepower — You need a given amount of horsepower to move a given amount of grain.
• Lifting capacity — Buckets will only lift so much depending on size, spacing and speed at which they are traveling.
The first item to consider in upgrading a leg is the drive unit. You need to make sure that your motor and gearbox are designed to handle the load that you anticipate the leg will need to move. Undersized drive units will not move enough grain and will result in the leg going down, needing to be scooped out and restarted.
Oversizing a leg can also be damaging. In this instance you transfer too much power to the leg, and something will be torn up. This could be the gearbox, the pulley, or even the belt. Also, by oversizing the drive, you continue to pay for it with higher than necessary utility bills.
PLASTIC OR STEEL?
The next item normally considered when upgrading a leg is the lifting capacity. Given the trunking, headpulley and tail pulley sizes, what are the bucket sizes that can be used on the belt? The trunking is going to limit the size and rows of buckets that can be used on the leg.
Next decide whether to go with plastic or steel. The obvious problem with steel is that if it hits concrete there will be a spark, which could be an ignition source. However, there are other factors to consider.
Different manufacturers will have different optimum speeds for their buckets. For the size that you choose, what are the different speeds and how will this affect the amount of grain legged? Steel and plastic discharge differently and will need to be run at different speeds to get the optimum discharge.
The holding capacity is different for steel and plastic. For a given size of bucket, steel will carry more grain and result in a higher throughput than plastic if all things are equal.
When changing from steel to plastic cups, remember that plastic cups have about a half-inch greater projection than steel. If there are projections in the leg casing, this may cause the leg to snag and break.
Steel buckets weigh two and a half to three times as much as plastic. If you have the same size of buckets and the same spacing, you will have two and a half to three times as much weight hanging on the head pulley with steel buckets than with plastic.
For example, with a 150-foot tall leg with 14 by 7 buckets on 9-inch centers, you will have 2,800 pounds (1,260 kilograms) hanging on your headshaft with steel, versus 1,100 pounds (495 kg) with plastic. When the buckets are full, you can see that there will be much less downward pull on the empty side of the leg as compared with the full side. This difference in weight will make it much easier to start slippage on the head pulley with plastic buckets, and this slippage will result in heat buildup.
To overcome this difference in weight you must run your belt much tighter with plastic than with steel buckets. This puts much more stress on your belt and splice. Belt strength needs to be considered with the tighter running belt. Belt splices also need to be considered. Most older facilities in the U.S. were installed with 36-inch tail pulleys while the head pulleys were 60 to 84 inches in diameter.
BELTS AND PULLEYS
When you are upgrading a leg, the belt normally needs to be heavier than previously. When you use a lap or butt splice on this heavier belt and it has to bend over the 36-inch tail pulley, it works the bolts severely on the splice from all of the flexing. This results in what is called a compression failure. All of this flexing, coupled with the inside belt flexing more than the outside one, results in the bolts that hold the splice together snapping.
I prefer the mechanical splice. It consists of four pieces of metal that bond the belt outward 90 degrees from its normal direction The belt is always in this position and does not flex back and forth; the resulting splice is strong and reliable.
With an older leg and steel buckets, the buckets provided enough weight that the tail pulley was used to keep the bolt tracking properly. In looking at the example given earlier, you can see that the tail pulley will need to provide an extra 1,700 pounds (765 kg) of force to get the same downward force with plastic as with steel buckets. This force is transferred through the tail shaft and bearings. You need to make sure that they are designed to handle the additional load, otherwise a failure by the shaft or bearings will result in a potential ignition source. A high percentage of elevator explosions can be traced back to bearing failures.
The same considerations are needed for the head pulley as the tail pulley. Are the components sized to handle the loads to which they will be subjected? The head pulley itself also needs to be checked. Was it designed to handle the required speed and capacity?
When doing the upgrade, check to make sure that the crown is still there. Having a good crown will minimize problems of the leg belt walking from side to side.
The lagging should also be checked to insure that it is in good shape or replaced at this time. Poor lagging will result in slippage, which loses capacity and develops heat buildup. Good lagging is always worth the time to install; lagging that may be faster to install often won’t last as long.
PRE-START UP CHECKS
Prior to starting up your leg, you need to go through further safety checks. First check the rotation of your motor. If your motor is running backward, it will tear out your backstop immediately and make it useless.
Gearboxes are shipped dry and need to be filled. Make sure that it has been filled to the appropriate level. Bearings need to be greased and lubricated, and couplings, where applicable, may need to be filled with oil or greased.
Double check the splice. Are all of the bolts tightened and in place? Also, the bucket bolts need to be checked.
Next, check your safety system. Is your alignment monitor working, and have you verified it? Has your speed detector been recalibrated for the different leg running speed? Are your temperature alarms reconnected to your bearings?
If you do not have an electronic safety system, then you need to manually verify alignment, speed/slippage and bearings and make sure that they are logged. If it is not recorded, it is not considered done.
The best safety program of all is your people using their senses:
Sight — Look at the equipment. Burned arcs on the trunking signify belt rubbing. Repaint area once belt is tracking properly.
Hearing — Listen to the equipment. It will tell you when something is going wrong.
Smell — Your nose can tell you of problems in grain long before it becomes evident through other means.
Touch — Feel for temperature differences and vibrations.
Communication pulls all of these together. The ability to communicate problems you see developing is the biggest contribution to making your facility a safe one.
The article is extracted from a presentation at the GEAPS Exchange 1996 by Tom Lechtenberg, director of station operations, Collingwood Grain, Inc., in Hutchinson, Kansas, U.S.