Hydraulic Jack and Cylinder Types, Installation Principles

Hydraulic Jack and Cylinder Types

Hydraulic elevators use a jack-and-cylinder assembly to raise and lower the car. The cylinder is a steel tube that holds hydraulic fluid under pressure; the jack (also called the plunger or piston rod) slides inside it and pushes the car upward. Three principal configurations are used in commercial and residential construction, each suited to different building conditions.

In-Ground (Conventional) Jacks

The most common arrangement buries a single steel cylinder vertically in a drilled or bored hole beneath the pit floor. The jack extends upward through a casing pipe that protects the cylinder from soil contact and groundwater. Because the cylinder descends below grade, the pit itself can be relatively shallow — the travel length is accommodated underground rather than above the pit floor. This makes in-ground jacks attractive when overhead clearance or pit depth above grade is limited.

Installation requires that the borehole diameter provide adequate clearance around the casing, typically several inches on all sides, with grout or sand backfill used to stabilize the casing and protect against corrosion. A containment system — usually a secondary casing or a sealed inner liner — is required by many jurisdictions and by ASME A17.1/CSA B44 to prevent fluid from migrating into the soil in the event of a cylinder leak. Installers must verify local environmental requirements before drilling.

Above-Ground (Holeless) Jacks

Holeless jacks eliminate the drilled borehole entirely. Instead, the cylinder is mounted above the pit floor. Two common holeless designs exist:

Single-bottom holeless: A single cylinder stands upright in the pit with its base resting on or anchored to the pit floor. The jack extends straight upward to push the platform. Because the full stroke length must fit within the pit and hoistway, pit depth requirements are greater than for in-ground systems on equivalent travel.

Twin-post (dual-cylinder) holeless: Two cylinders are mounted side-by-side beneath the car platform, one on each side. This spreads the load symmetrically and allows each cylinder to be shorter, reducing the required pit depth compared with a single above-ground unit. Twin-post systems are common in low-rise retrofits and locations where drilling is prohibited by rock, contaminated soil, or underground utilities.

Because holeless cylinders are fully accessible, inspecting and servicing seals is simpler than on in-ground units. No containment casing is required, reducing environmental compliance complexity.

Roped-Hydraulic Jacks

A roped-hydraulic system combines a hydraulic jack with a sheave-and-rope arrangement similar to traction elevator rigging. The jack extends horizontally or at a low angle, or a shorter vertical jack drives a sheave; wire ropes attached to the car platform travel over the sheave and multiply the stroke. This allows full travel heights that would otherwise require an impractically deep cylinder or borehole. Roped-hydraulic elevators are used when travel exceeds the practical length of a direct-acting jack and when the building cannot accommodate a traction machine room above.

The rope and sheave arrangement introduces additional components — hitch plates, sheave bearings, and rope tension — that must be inspected and maintained alongside the hydraulic components.

Installation Clearance Requirements

Regardless of jack type, installers must observe clearance rules during rough-in and final installation. The cylinder centerline must align with the guide rail centerline and the sling/platform guides to prevent side loading on the jack. Side loading accelerates seal wear and can score the cylinder bore. Plumb and alignment checks using a level and plumb bob are essential before any grouting or anchoring is finalized.

For in-ground jacks, the casing must extend above the pit floor by the dimension specified in the approved plans, and the pit must be kept dry during installation to prevent the borehole from collapsing. For holeless jacks, anchor bolts must be torqued to the manufacturer's specification and verified before the system is put into service.

Jack Seals and Packing: Preventing Fluid Loss

The packing gland (also called the stuffing box) is the assembly at the top of the cylinder through which the jack rod slides. It contains a stack of sealing components that together prevent hydraulic fluid from escaping along the moving rod surface.

Wiper seal (dust wiper): The outermost seal. As the jack retracts into the cylinder, the wiper scrapes dirt, moisture, and debris from the rod surface so that contaminants do not enter the cylinder. A damaged or worn wiper allows particles to migrate inward, cutting the inner seals.

Primary packing (O-ring or lip seal set): The main pressure-retaining element. These rings are compressed against the rod and cylinder bore to block pressurized fluid from moving past the gland. O-rings provide a static or low-movement seal; lip seals handle dynamic motion better and are common in modern packing kits.

Backup ring: Positioned behind the primary O-ring to prevent it from extruding into the clearance gap under high pressure. Without a backup ring, the O-ring can deform and be destroyed rapidly when system pressure spikes.

Seal failure presents in recognizable patterns: a leaking wiper alone causes a thin oil film or drips on the rod exterior; a failed primary packing produces a steady weep or flow of oil around the gland; backup ring failure typically accelerates primary packing failure and appears as frequent seal replacements needed in a short period. All hydraulic fluid leaks must be corrected promptly — fluid on the pit floor creates a slip hazard and an environmental violation risk.

Safety Points

Before any work on a jack or packing gland, the system pressure must be fully relieved and the car must be blocked or cribbed so it cannot descend. Apply lockout/tagout to the pump motor and the electrical disconnect per your site's energy-control program. Wear safety glasses and fluid-resistant gloves when handling hydraulic oil. Dispose of used oil and seals in accordance with local environmental regulations. Never work beneath an unsupported car.