Wall Thimbles Spec Sheet
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Types of Thimbles
Thimble types derive their names from the shapes of their cross sections. The most frequently used thimble, and Hydro Gate’s standard, is the F-type. The front machined face (top of the F) mates with the gate frame and holes are drilled and tapped to receive studs for holding the gate to the thimble. The small staff of the letter F acts as a cleat to hold the thimble in the wall under back pressure conditions. Except when gates are subjected to a very high unseating head, this cleat on the F is adequate to distribute the load evenly to the concrete. It also forms a water stop to prevent percolation of water around the thimble.
A thimble similar to the F is the E-type. Another full flange is added to the F and used when the back of the thimble is attached to an extension liner, another flange, a trash rack, or a flap gate. The E-type thimble may also be required when slide gates are subjected to very high unseating heads.
Mechanical joint thimbles are available in round openings for attachment of ductile iron pipe. Fasteners, packing, and follower rings are available from pipe suppliers. Another variation of the mechanical joint thimble is the push-on joint type (bell & spigot). These are seldom used due to the wide variation in sizes. It is to the user’s advantage to stay with the most common type of thimbles, which are the F, E, and mechanical joint types. Other types often require special patterns, pattern modification and difficulty in specifying dimensional details lead to longer waiting periods for deliveries.
Hydro Gate furnishes, as standard, one or two vent holes at center of the barrel on square or rectangular thimbles 30 in. and wider to vent air out of entrapment zones. If grout holes on 2 ft. centers or thread holes for pressure grouting are desired, they should be specified by the owner or consultant.
Standard lengths are 12 in. to 18 in. and use of standard lengths will facilitate early delivery.
Thimbles are stamped “Top” and have the vertical centerline located to aid in alignment.
Because a thimble is an entirely separate part, it can be shipped prior to the gate so that it can be included in the construction forms before the concrete is poured.
A thimble has a machined flange for mounting of the gate. Special thimbles, such as E-type and the mechanical joint (MJ) thimbles, have surfaces for attaching pipe or equipment on the other end. Embedment of the thimble results in an iron-lined opening and a flat machined face
for gate mounting. Thimbles provide a solid mounting for gates under high heads to resist the hydraulic and operating forces to which gates are subjected. Mounting a gate directly on concrete with anchor bolts and a sealing grout pad is not satisfactory for moderate and high heads, since the dynamic loads can cause fatigue failure of the grout. Reasonable care must be exercised in forming, aligning, and bracing of the thimble to prevent warping of the flange face during concrete placement.
When a thimble is used, nearly all anchor bolts are omitted. Only those anchor bolts needed above the gate opening to support the top ends of the gate are required. Problems associated with mounting a gate directly on concrete can be avoided with thimbles. These problems include:
- The large amount of time required to place and secure anchor bolts in the forms;
- The likelihood of placing anchor bolts in the wrong location; and
- The need to realign anchors after removal of forms.
When a thimble is used and the gate arrives on the job, a good machined surface is already in place for attachment of the gate flange. Studs are used for attaching the gate frame to the thimble. These are quickly installed and the back of the machined frame is pulled up against the front face of the thimble. This simple procedure eliminates the time-consuming job of installing the gate on anchor bolts, aligning it, and grouting behind the frame.
A thimble permits the gate to be removed and transferred to another location for cleaning and painting. Future gate installations can be provided for by placing thimbles when the concrete is poured, then covering the openings with blind plate flanges until the gates themselves are required.
There are two options for sealing the joint between the face of the thimble and the back of the gate frame. The first option is to apply a mastic between the machined surfaces. Excess mastic is squeezed out as nuts on studs are uniformly tightened until the flanges are metal to metal. This thin film of mastic provides a watertight joint between the machined flanges. The second option is to use a 1/8-in. rubber gasket, but extra care must be taken to obtain uniform tightening of nuts on the studs. Otherwise, warping of the gate frame may result.
Cloth-inserted gaskets are not recommended, as the fabric has a tendency to rot.
Lead gaskets are not recommended because the thickness of the lead sheet is usually not uniform, and lead is not environmentally acceptable.