WHAT IS AN ELEVATOR JACK?

There are two general types of elevators – hydraulic and traction.  This article is focused on hydraulic elevators with a conventional hydraulic jack. The hydraulic jack is located in the pit, underneath the elevator car and consists of the hydraulic cylinder and piston. It is connected to a power unit, located in the elevator machine room, by a pipe/oil line. The power unit consists of a pump, motor, valve, muffler, and oil reservoir (tank). To move up, the motor turns the pump, the valve allows the oil to flow, which pressurizes the system. The piston extends out of the cylinder, pushing the elevator car up. To move down, gravity does the work. The valve allows the oil to flow back into the tank, the piston retracts into the cylinder and takes the car with it. A conventional jack is one piece. People are often amazed to learn that the jack goes into the ground as deep as it the elevator travels. If your elevator travels six stories, it goes into the ground that far. For that reason, hydraulic elevators are ideal for low- to mid-rise applications. They typically max out at eight stories, although there are a handful of elevators in Seattle that are ten stops or more (17, according to City records). Less common types of jacks exist as well – above ground, twin post, telescoping, roped hydraulics. For now, we are going to focus on conventional in-ground jacks.

ELEVATOR JACK HISTORY

Single-bottom jack   The earliest hydraulic jacks were essentially a round steel pipe with a plate welded on the bottom, often referred to a “Single-bottom jack”. These were installed within a hole and backfilled with dirt. The weld that attaches the bottom plate to the cylinder wall is susceptible to corrosion and leaks. A leak or complete failure can result in uncontrolled descent of the elevator. This creates serious safety concerns. 

Double-bottom jack.  To address the safety concerns posed by the single-bottom jack, the 1971 version of ASME A17.1 code introduced a requirement for a safety bulkhead, or “double-bottom jack”.  The safety bulkhead moves the welds to an area on the cylinder that is less susceptible to failure.  And more importantly, if it does fail, the safety bulkhead has an orifice that limits the speed of the car’s descent, minimizing the safety risk. 1971 code was adopted in Washington in 1972. Anything installed after that code adoption should have a double-bottom jack. 

Double-bottom jack.

PVC  Code was revised again in 1989 to require a PVC sleave. PVC is designed to protect the steel cylinder from ground moisture. Because moisture is what causes jacks to fail, installs after 1989 code should last indefinitely. Washington adopted 1990 code, the first version after 1989, in 1993.

This progression of changes has improved the overall safety of the equipment.

ELEVATOR JACK FAILURE

Part of any elevator maintenance program will include the hydraulic fluid monitoring, which is logged in the Maintenance Control Program (MCP). The most common instance of returning oil to the tank is from collection containers, pans, and seals or packing glands. Most elevators have a bucket that collects oil which leaks from the jack packing. Whenever this oil is returned to the system, it is logged in the MCP.

If oil loss cannot be accounted for, the elevator must be removed from service until the source of the leak can be identified. Typically, the elevator will be taken to the top floor and the oil level will be marked. All shutoff valves will be closed (if applicable). After a period of time (overnight), a mechanic will return to check the oil levels. If the elevator drifts down and the oil did not transfer back into the tank, it is possible that the jack has a leak. Any time oil is added/missing in the system, it will be recorded in the MCP.

ELEVATOR JACK REPLACEMENT

Whether your jack has a leak, or you decide to proactively upgrade it, there are a few variables that can have a huge impact on the project cost and duration. Most involve things below ground. I will touch on some of the more common variables later. First step in a jack replacement is removing the existing jack.

Removal

• The car is secured at the top floor, where it will remain for the duration of the project. 

• The piston is detached from the bottom of the car.  Since it is not normally reused, it is generally cut into pieces to get it out. 

• Pit equipment (buffers, pit channels, piping) are removed.

• The concrete pit floor is chipped away to access the cylinder. 

VARIABLE #1

This is often when we learn about the first major variable – the casing. Jacks were sometimes installed within a steel well casing. Having a well casing that is centered, plumb and goes the entire depth of the cylinder is the best-case scenario. Sometimes casings were installed for just a portion of the depth or not at all. If a casing exists, the cylinder can be easily hoisted out of the casing. If there is no casing, removal can be more difficult. We still start by trying to hoist it out, but depending on the resistance, other methods may be needed.

Even with a well casing, sometimes the bottom needs to be cleared. This can be accomplished by using a vacuum truck.

VARIABLE #2

If there is no well casing, or the existing well casing is not centered, plumb or deep enough, a new casing will need to be installed. This is accomplished by drilling a larger hole. A larger diameter will also accommodate the new PVC that is needed. We partner with DBM for drilling. They can be hired directly or subcontracted. If subcontracted by an elevator company, the elevator company must provide a standby mechanic during their work, adding a significant cost.  DBM uses a drilling rig that mounts to the elevator rails. The soil or “spoils” removed in drilling the hole must be disposed of. Prior to disposal, it must be tested for contaminants. If contaminants are found, special arrangements must be made.

Each project is unique. Variables are usually presented in the removal or drilling phase of the work. Sometimes the jack is not easily removed. Drilling can go slow. Or water can be present. They all add to the cost and downtime, something you should be prepared for.

Jack Installation  Once there is a plumb casing, jack installation can commence. Most variables are removed at this point. 

• PVC sleave is lowered into the hole.

• Hydraulic cylinder is lowered into PVC. It often comes in multiple pieces and is assembled at this time. 

• The piston is assembled and installed into the cylinder. 

• Concrete is poured around the cylinder.

• Pit equipment (buffers, pit channels) are installed. 

• Piping is connected. Two shutoff valves, one in the machine room and one in the pit, and one overspeed valve (AKA rupture valve) are installed if the elevator did not already have them. 

• The elevator valve will be adjusted.

• Category 5 test performed. 

• Inspection with AHJ. 

Duration 

If a casing exists and no variables are encountered, a typical installation takes between two and three weeks. Drilling can add four to five weeks (or more) to the project, not including scheduling and mobilization. 

CAN A JACK BE INSPECTED?  Unfortunately, no. I often get this question. The jack is installed into the ground, sometimes up to eight stories deep. There is no way of knowing the condition of the jack, or whether a casing is present. Sometimes a casing is visible in the pit floor. This can indicate that there is a casing for part of the depth. But this is not a guarantee that it goes the depth or is plumb.  

SHOULD I REPLACE MY HYDRAULIC JACK?

It is generally best to be proactive when replacing a hydraulic jack to minimize downtime. Waiting for failure will add months to the downtime for testing, bidding, material procurement, and scheduling (DMB is often months out). Here are a couple considerations:

• Failure.  If your jack has failed, you absolutely must replace it. 

• Single-bottom jacks.  If you have a single-bottom jack based on the install date, you should be making plans to replace for safety/liability reasons. 

• Double-bottom jacks.  If your double-bottom jack was installed before PVC is required, it can still fail. The safety concerns are minimized (but still there), but downtime in the event of failure can be excessive. If PVC was installed, there should be no reason for replacement. 

• Modernization.  If you are planning to modernize, this is a good time to consider replacing the jack. If you have a single-bottom jack, it must be replaced during the modernization. Double-bottom jacks are not required to be replaced, but if they do not have PVC, it is worth considering.  Experiencing a failure after the cost and downtime of a modernization would be unfortunate and frustrating. The cost of upgrading during the modernization will be less due to economies of scale. 

Questions? Call your local expert Chief Problem Solver, Sean McMannis!