Elevator jackshafts overview
Elevator Jackshafts are shafts used to hold hydraulic jacks for elevators. Hydraulic jacks are the most common mechanical device for lifting elevators, representing around 70% of elevators installed in new building structures. Elevators of this type utilize a single plunger jack installed underneath the elevator car. Hydraulic jacks are ideal for low and medium rise buildings (typically 2-5 floors), given their low mechanical complexity. Elevator Jackshafts can reach depths up to 75 feet.
The cost of installation for an Elevator Jackshaft and hydraulic jack are approximately half the cost of a traction elevator, and utilization of underground space to hold the jack allows for more efficient use of space, as an overhead machine room is not required.
In the installation of an Elevator Jackshaft, a 24″ minimum diameter steel casing is placed inside an oversized drilled hole. The casing is fitted with a water-tight bottom plate, so as to prevent groundwater from seeping into the inner chamber. The casing is then placed within plumb and line tolerances. The space between the outside of the casing and the shaft is backfilled with displaced soil from the drilled hole for stability. If the displaced soil cannot be used for backfilling, the prime contractor or owner must provide granular backfill material at their own expense. The Elevator Jackshaft is then inspected, and afterwards a temporary cover is welded to the top of the casing, to be removed when the final welds for the elevator jack are installed.
The casing in an Elevator Jackshaft is required because the material within the casing must stabilize the jack for heavy loads. Grout and sand are suitable for supporting the jack inside the casing, while compressible soil can shift, reducing the integrity of the jack. In cases where the Elevator Jackshaft penetrates a rock layer, a casing may not be required along the rock layer segment. Given correct installation, hydraulic jacks are most vulnerable to chemical and electrolytic corrosion, which can cause leakage of the jack’s internal oil or casing. Special wrappings can be used on the jack to isolate it from outside fluids or soil that causes deterioration.