Silo Bolts |
hoosing the Correct Size Silo Bolt
For maximum strength of the joint, it is important to select the correct length of bolt. Longer bolts have a shank, and if the shank is too long, this will stop the nut tightening before the joint is fully closed, and the bolt will spin, damaging the glass coating; a bolt that is too short will not have enough thread for the nut to grip properly. In the joint, the shank of the bolt should be in shear, not the threaded part of the bolt. To work out the length of silo bolt required, measure the thickness of all the pieces that will make up the joint. Add these thicknesses together, and consult the chart below.
E.g. The vertical seam in the top ring of a silo: 2.0 mm sheet +2.0 mm sheet = 4.0mm. From the chart below, a 1” bolt would be used here.
Another example: the base angle to the bottom ring of sheets, at the corner joint between two sheets of a slurry store: 4 mm sheet+ 4 mm sheet + 6 mm angle = 14mm. Using the chart again, a 1½” bolt should be used here.
Total Thickness of Material Length of Bolt Required
4.0mm to 7.5mm 1” bolt, grade 8.8
8.0mm to12.5mm 1¼” bolt, grade 8.8
13.0mm to18.0mm 1½” bolt, grade 10.9
18.5mm to 25mm 1¾” bolt, grade 10.9
25mm to 32mm 2” bolt, grade 10.9
33mm to 45mm 2½” bolt, grade 10.9
45mm to 60mm 3” bolt (full length thread)
For maximum strength of the joint, it is important to select the correct length of bolt. Longer bolts have a shank, and if the shank is too long, this will stop the nut tightening before the joint is fully closed, and the bolt will spin, damaging the glass coating; a bolt that is too short will not have enough thread for the nut to grip properly. In the joint, the shank of the bolt should be in shear, not the threaded part of the bolt. To work out the length of silo bolt required, measure the thickness of all the pieces that will make up the joint. Add these thicknesses together, and consult the chart below.
E.g. The vertical seam in the top ring of a silo: 2.0 mm sheet +2.0 mm sheet = 4.0mm. From the chart below, a 1” bolt would be used here.
Another example: the base angle to the bottom ring of sheets, at the corner joint between two sheets of a slurry store: 4 mm sheet+ 4 mm sheet + 6 mm angle = 14mm. Using the chart again, a 1½” bolt should be used here.
Total Thickness of Material Length of Bolt Required
4.0mm to 7.5mm 1” bolt, grade 8.8
8.0mm to12.5mm 1¼” bolt, grade 8.8
13.0mm to18.0mm 1½” bolt, grade 10.9
18.5mm to 25mm 1¾” bolt, grade 10.9
25mm to 32mm 2” bolt, grade 10.9
33mm to 45mm 2½” bolt, grade 10.9
45mm to 60mm 3” bolt (full length thread)
Using Silo bolts
The head of the bolt is always inside the tank or silo, with the nut and washer outside. A blob of tank grade mastic (usually Sikaflex TS+) is applied over the hole before the bolt is pushed through, which will act as a cushion between bolt head and sheet, and seals the underside of the bolt head to the inner surface of the tank, thus protecting the bolt from corrosion by very aggressive chemicals. An erector on the outside places the washers on the protruding bolt, in a manner that sweeps the remains of the blob of mastic under the washer, to act as a cushion against the sheet surface. The external erector then puts the nut on to the end of the bolt, just giving it about half a turn, enough to stop it falling off. The bolts are then tightened with a nut runner to a torque of 65Nm. Where the sheet overlap is covered by a base or top angle, the nut next to this joint is only tightened to 32Nm; a tapered insert is usually fitted here. The bolts are tightened in an order that makes the joint evenly, so that the silo sheet is not stressed, which could damage the coating reducing the life of the silo or tank. The video below shows a good sequence of tightening. Alongside is a sectional view of a correctly made joint. There is a cushion of mastic under the bolt head and washer, and the edges of both sheets are protected by mastic. Naturally, the joint between the sheets is completely filled with mastic. The shank of the bolt, rather than the threaded portion, is in shear.
When should silo bolts be replaced?
When a silo or tank is rebuilt, all bolts, nuts and washers should be replaced as a matter of course. Using old bolts and nuts wastes time, and interrupts the rhythm of a good erection team.
All silos and slurry stores should be inspected from time to time. We recommend every five years. * The most common fault with silo bolts will be degradation of the plastic cap due to UV light. This frequently occurs in slurry tanks, usually affecting the heads of bolts in the top ring(s) only, as these are not always covered with the slurry. This will be noticeable after about 15-20 years.
When a silo or tank is rebuilt, all bolts, nuts and washers should be replaced as a matter of course. Using old bolts and nuts wastes time, and interrupts the rhythm of a good erection team.
All silos and slurry stores should be inspected from time to time. We recommend every five years. * The most common fault with silo bolts will be degradation of the plastic cap due to UV light. This frequently occurs in slurry tanks, usually affecting the heads of bolts in the top ring(s) only, as these are not always covered with the slurry. This will be noticeable after about 15-20 years.
Replacement is fairly straightforward, but of course it is essential to test for poisonous gases before entering a slurry store, and carry a gas alarm at all times. For safety advice about slurry gas click here
* We will be happy to inspect your slurry store, providing a full written report on the tank’s condition, with any recommendations for remedial action .
Nut Caps
Some customers may ask for the nuts to be capped. These caps cover the exposed thread, the nut and the washer. This is purely cosmetic; we can supply black plastic nut caps to special order.
* We will be happy to inspect your slurry store, providing a full written report on the tank’s condition, with any recommendations for remedial action .
Nut Caps
Some customers may ask for the nuts to be capped. These caps cover the exposed thread, the nut and the washer. This is purely cosmetic; we can supply black plastic nut caps to special order.