Wall Thimbles

Description
A wall thimble is an entirely separate part from the gate proper
and is made of cast iron. Its front face is machined and drilled
for attachment of a gate frame. Its spigot runs parallel to the
direction of flow and is of varying length depending upon the
thickness of the concrete wall. Thimbles are embedded in
concrete, preferably in the original pour, and are one-piece
castings.

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
Square flange with mechanical joint wall thimble 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
Features 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.
n Cast Iron (See figure 4-1, p. 31).
n Machined Faces, Drilled, and Tapped
Mechanical joint thimbles are available in round openings for
for Gate Attachment attachment of ductile iron pipe. Fasteners, packing, and
follower rings are available from pipe suppliers.
n Forms Opening Through Wall
Hydro Gate furnishes grout holes along the bottom barrel
surface of all square and rectangular wall thimbles with widths
greater than 30 inches. Thimbles with round barrels do not
require grout holes and are not provided for standard
applications. If custom grout holes or vent connections are
required, please contact Hydro Gate to discuss options.

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.

C A S T I R O N S L I D E G A T E 23
Advantages 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
Because a thimble is an entirely separate part, it can be
flange. Studs are used for attaching the gate frame to the
shipped prior to the gate so that it can be included in the
thimble. These are quickly installed and the back of the
construction forms before the concrete is poured.
machined frame is pulled up against the front face of the
thimble. This simple procedure eliminates the time-consuming
A thimble has a machined flange for mounting of the gate.
job of installing the gate on anchor bolts, aligning it, and
Special thimbles, such as E-type and the mechanical joint (MJ)
grouting behind the frame.
thimbles, have surfaces for attaching pipe or equipment on the
other end. Embedment of the thimble results in an iron-lined
A thimble permits the gate to be removed and transferred to
opening and a flat machined face for gate mounting. Thimbles
another location for cleaning and painting. Future gate
provide a solid mounting for gates under high heads to resist
installations can be provided for by placing thimbles when the
the hydraulic and operating forces to which gates are
concrete is poured, then covering the openings with blind plate
subjected. Mounting a gate directly on concrete with anchor
flanges until the gates themselves are required.
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 Sealant
forming, aligning, and bracing of the thimble to prevent There are two options for sealing the joint between the face of
warping of the flange face during concrete placement. the thimble and the back of the gate frame. The first option is
to apply a mastic between the machined surfaces. Excess
When a thimble is used, nearly all anchor bolts are omitted. mastic is squeezed out as nuts on studs are uniformly tightened
Only those anchor bolts needed above the gate opening to until the flanges are metal to metal. This thin film of mastic
support the top ends of the gate are required. Problems provides a watertight joint between the machined flanges. The
associated with mounting a gate directly on concrete can be second option is to use a 1/8-in. rubber gasket, but extra care
avoided with thimbles. These problems include: must be taken to obtain uniform tightening of nuts on the
studs. Otherwise, warping of the gate frame may result.
1. The large amount of time required to place and secure
anchor bolts in the forms; Cloth-inserted gaskets are not recommended, as the fabric has
2. The likelihood of placing anchor bolts in the wrong a tendency to rot.
location; and
3. The need to realign anchors after removal of forms.

Figure 4-1
Typical Wall Thimble Sections

24 C A S T I R O N S L I D E G A T E
Types of Openings
Square Flange – Square Opening Thimbles for Attaching Gate and Pipe
and Rectangular Flange – Hydro Gate can furnish two styles of special thimbles to
Rectangular Opening facilitate attachment of incoming or outgoing pipelines from
treatment plants. Both styles utilize the square front flange for
These thimbles, used with square or rectangular opening slide
attaching the square slide gate. The opening through the
or flap gates, are commonly of the F-type, supplied in standard
thimble and wall is converted to round to match the pipe. The
lengths of 12 in. and 18 in., depending on the gate size and
other end of the E-type thimble has a round flange which can
wall thickness.
be drilled to match the commercially available flange for
various cast iron, steel, or concrete pipes or pipe made from
Square Front Flange – Round Opening other materials. This round flange is drilled and tapped for
Locations requiring round openings through the wall should ease of field assembly to the flange on the pipe. The length of
utilize this type of thimble. The front face is cast with a square the E thimble can be varied to match the thickness of the wall
flange to receive the square gate. The opening through the and thus avoid any other special concrete forming or
thimble is round. All four corners are cast with webs of iron to adaptation.
convert from the square flange to the round opening. Four
reinforcing ribs are cast on the back of the thimble to give the Mechanical Joint Thimbles
corners the stiffness and strength required. This reinforcing is
The mechanical joint end of the thimble is bored to receive
embedded in the wall when the concrete is poured.
ductile iron pipe, packing and followers. The end is drilled and
tapped for studs.
Gate installation is simplified with the square flange. All studs
are around the outside square where they are readily
accessible. When a flange-back gate is used, and installation
clearances are to be kept at a minimum, studs can be projected
through the frame of the gate to reduce side and bottom
clearances needed.

Round Front Flange – Round Opening

This thimble consists of a round flange attached to the round
opening portion of the thimble. It is recommended with round
opening flap gates only. Openings for slide gates are best
provided through use of square flange, round opening
thimbles.

Round flange-back gates can be attached to round flange

thimbles, but much larger clearances along the side and
bottom of the gate and wall thimble are necessary.

Square flange with mechanical joint wall thimble.

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