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Selection of Lifting Devices
In order to operate any gate, the lifting device must overcome several forces. These include the weight of the gate slide, weight of the stem, frictional resistance caused by water pressure against the slide, the frictional resistance caused by the wedges and hydraulic downpull.
Required Lifting Force
To determine the lifting force required to open a gate, the following formula is used:
F + 62.4APf+W+w+dp
F = lifting force required in pounds
A = area of gate opening in square feet
P = effective head of water in feet
f = coefficient of friction
W = weight of gate slide in pounds
w = weight of stem in pounds
dp = downpull in pounds
After the gate has been unwedged, the coefficient of friction is a conservative value of approximately 0.35. As indicated, frictional factors are approximate and will vary depending upon how long the gate has been in the closed position; if the slide is partially covered by silt or sand; if the faces are lubricated or dry; and the condition of the threaded portion of the stem.
The area of the square or rectangular gate opening is used to determine the frictional load created by the water pressure on either the face or the backside of the gate. The area of the square is also used to determine frictional load for round sluice gates since they are made with square slides with the seating faces mounted on a square around the circular opening. Therefore, the water pressure is always active against the square area.
For roller gate operation, the same formula is used to determine the lifting force required, except the frictional factor is 0.2. This smaller factor is adequate because this type of gate rolls on rails and has no wedging device. Fabricated slide gates have a frictional factor range of 0.15 to 0.35 depending on type of seal and guide liner used.
The force to unwedge, unseat or crack open a gate is an instantaneous force. Experience indicates that this instantaneous friction factor is normally no more than double the running friction or 0.70.
Hydraulic downpull is a factor based on head, flow conditions, geometry of the gate slide, structure entrance, exit shapes and gate position. Consult Hydro Gate’s Engineering Department for further details.
Design of Lift Platform
To start or lift a sluice or slide gate from the fully closed position, the platform or other mounting for the lift support device must be designed to take the thrust with the minimum amount of deflection.
Selection of Proper Lifts
Capacities for lifting devices are shown for 50 ft. lb. Input torque (40 lb. Pull) on the crank or handwheel (see table on page 5-10. Lifting devices selected on the basis of a 25-lb pull will be easier to operate, but the larger the lift, the more turns of the crank per inch of gate travel will be required. Regardless of whether the lifting device is selected on the basis of a 25- or 40-lb pull, the coefficient of friction 0.35 is used for lift selection. The crank or handwheel pull is approximately double that to “crack” the slide from its wedges. The higher pull is only required for a few turns of the crank. It then drops back to the 25-or 40-lb average.
There are two basic steps in selecting the lift and stem. The first step is to determine the total lift load and the minimum permissible stem diameter that can be used with any given gate size. The tables on pages 5-7 and 5-8 give theoretical combined weight plus downpull values for a wide range of gate sizes operating under unbalanced heads of 5 to 60 ft. After the gate size has been selected and the maximum unbalanced operating head has been calculated, use these tables to determine the lift load.
The next step is to determine the actual stem diameter required for the stem material selected. The “Allowable Loads” tables (page 5-9) show the maximum permissible loads for each size of stem for various unsupported stem lengths. In selecting the actual stem size, use a diameter that has a strength at least equal to the lift load obtained from the table, and a diameter equal to or greater than the minimum needed for column action in closing the gate.