The weight and pulley balance system for double-hung windows was, for many years, the standard in wood window construction. The inherent dependability of this system is once again bringing it into favor with quality-minded architects, builders and window fabricators. The purpose of this article is to examine some of the factors affecting the specification and design of weight and pulley balance systems for today's windows.
The basic principle of a weight and pulley balance system is a simple one: A vertically sliding sash is attached to and suspended by lengths of cord or chain. Each cord or chain passes over a pulley mortised into the window jamb near the top of the opening. The opposite end of each cord or chain is attached to an iron, steel or lead counterweight weighing one-half the weight of the sash it is balancing. The counterweights are located within a concealed space or weight pocket. An accurate and well thought out design will result in sash which require little or no effort to operate and which will remain open or closed as desired.
There are four important components to a properly designed weight and pulley window balance system:
The specifications of a weight and pulley balance system are directly affected by the dimensions and weight of the window sash. The manner in which these two criteria affect the specification of the other component parts is outlined below:
Width: Windows which are wide in proportion to their height will be heavy in proportion to their height. A counterweight for a short, wide window must be heavy in proportion to its length because the vertical space available for the travel of the weight is limited.
Thickness: Since the counterweights for the upper and lower sashes of a double-hung window operate side by side within the pocket, the sash thickness places a limitation on the cross-sectional dimensions of the counterweights. Generally speaking, a window with 1 3/4" thick sash will use 1 3/4" round or square counterweights. If 1 3/4" weights were used on a window with 1 3/8" thick sash, the weights would collide during their vertical travel. So the maximum cross-section of sash weights is determined by the sash thickness.
Glazing: The largest percentage of the weight of a window sash is in the glazing. The type of glass specified will therefore have a direct effect not only on the weight of the sash, but also on its required counterweight. A sash glazed with sealed insulating glass will weigh nearly twice as much as the same sash with single glazing and, therefore, require a counterweight twice as heavy. It is critical to the system design that the thickness of the glass be specified at the outset and the finished sash weight calculated prior to the specification of the balance components. Any changes to the glazing specifications may require re-specification of the other components.
NOTE: Insulating glass is made up of two separate sheets of glass hermetically sealed. The thickness of each sheet must be specified so that the weight may be calculated. The overall thickness of the sealed unit does not materially affect its weight.
Calculating sash weight and sizing counterweights
The best and most accurate way to calculate sash weight is to weigh the completed and glazed sash. For design purposes, estimates can be made based on the approximate weight of wood, glass, and hardware. The total weight of the two sash counterweights should equal the weight of the sash being balanced. The amount of operational friction in the sliding sash will determine the allowable margin of error in sizing the counterweights. The operational friction is largely a function weatherstripping.
Laminated glass and insulating glass can push the limits of weight and pulley window design. This is especially true with designs involving the retrofitting of new insulating glass sash into existing window frames. Changing the glazing increases the sash weight dramatically, and it should be carefully verified that the existing weight pockets will accept appropriate counterweights.
In old work or new, the most challenging window to balance will be one whose width is large in proportion to its height. It is good practice in either new work or retrofit, to do the calculations for the window which is widest in proportion to its height. If calculations show that this sash can be balanced with counterweights that will fit and operate within available the weight pockets, all of the other windows should be possible as well, regardless of their size.
Some other factors to consider in system design are:
The thickness of the window frame side jamb in which the sash pulley is mounted, in conjunction with the sash pulley diameter, will determine how large the cross-section of a counterweight can be and still hang freely within the weight pocket. Jamb thickness should not be oversized in an attempt to create a heavy duty design. Jamb thickness over 1" will interfere with pulley operation. In the case of large and heavy windows, a box-jamb design should be used. Jamb thickness itself should remain at approximately 7/8".
Location (Height) of the Sash Pulleys
Locating the sash pulleys as high as possible within the sash opening increases the vertical space available to the counterweight for its travel within the weight pocket. However, locating the pulleys’ faceplates too close to the head jamb may make installation difficult. Locating the pulley faceplate 1 1/2" from the head jamb is a good rule of thumb.
Sash Weight Pocket Access
A removable panel should be incorporated into the window jamb to allow access to the sash counterweights. If one piece counterweights are used, the access panel size and height must be carefully calculated to allow removal or installation of the counterweights. An important advantage of sectional, or stacking counterweights, is the flexibility to use one size of access panel for all of the various window sizes.
In conclusion, it is useful to remember the reasons which contribute toward making the weight and pulley balance system a permanent and reliable mechanical system for the operation of double-hung wood windows:
Weight and Pulley Design Checklist and Calculation Examples A worksheet to assist in designing a weight and pulley system for wood windows. Includes two examples showing calculations for a typical system ... read