Jack

Overview

A car jack is a compact, portable lifting device that utilises a rigid lifting member to raise heavy loads within its travel stroke via a top saddle or base claw. Highly portable and lightweight in construction, it is an essential piece of on-board emergency equipment stowed in the boot of almost every passenger car, primarily used for roadside tyre changes, undercarriage inspections, minor repairs, and vehicle servicing.

Working Principle and Main Types

Depending on their operating mechanisms, automotive jacks are broadly categorised into two main types: mechanical and hydraulic.

Mechanical jacks rely on mechanical gear or screw drive systems to lift a vehicle, with the most common variants being screw jacks and scissor jacks. A screw jack turns a threaded drive screw to extend an internal sleeve upwards; it boasts a simple structure, low cost, and zero reliance on external power, but is comparatively slow and physically demanding to operate. Screw jacks typically feature a load rating between 5 and 50 tonnes, with an operating stroke of 100 to 150 mm.

Scissor jacks (also known as diamond jacks) operate by turning a threaded central rod that expands or collapses a diamond-shaped pantograph linkage, making them the standard factory-supplied emergency jack for passenger cars. They are lightweight (typically under 2 kg), fold down flat for easy storage, and are economical to manufacture. With rated capacities usually between 1 and 2 tonnes, they easily meet the tyre-changing demands of saloons, hatchbacks, and compact SUVs. However, their primary shortcomings include slow winding and limited lateral stability.

Hydraulic jacks operate on Pascal’s principle—pressure applied to an enclosed fluid is transmitted equally in all directions. When manual pumping actuates a small piston, hydraulic fluid forces a larger piston upward; the significantly larger surface area of the main piston multiplies the effort into immense lifting force. Offering high lifting capacities (up to thousands of tonnes in heavy-duty applications), smooth elevation, and fast operation, hydraulic jacks are the preferred tool in workshops and professional garages. Their drawbacks include potential seal leaks, regular maintenance demands, and an inability to safely hold elevated loads over prolonged periods.

In terms of design, hydraulic jacks are available in several configurations, including bottle jacks (vertical ram type), trolley/floor jacks (horizontal cylinder layout), and split/porta-power units (where the pump is separate from the ram). In professional workshop settings, bottle jacks are the most common—readily available in capacities of 2, 5, 8, 10, 20, and up to 50 tonnes, making light work of lifting a wide spectrum of vehicles.

Safety Operating Procedures

Improper jack usage remains a leading cause of workshop and roadside vehicle accidents. The following safety protocols must always be observed:

Selecting the correct jacking point is the most critical rule. A vehicle's chassis features reinforced factory jacking points (typically located along the sill seams behind the front wheels or ahead of the rear wheels, indicated by distinct notches or reinforced pads). The jack must align squarely with these designated spots. Jacking against weak components such as the engine oil sump, gearbox housing, or suspension control arms will cause severe mechanical damage.

Ground conditions: Jacks must strictly be operated on flat, firm, and level surfaces. If jacking on unpaved ground or an incline, place a solid timber plank or heavy-duty metal spreader plate beneath the base to distribute the load and prevent sinking or slipping.

Secondary support: A jack must never serve as the sole support for an elevated vehicle. Once raised to the desired working height, immediately secure the vehicle with jack stands (axle stands) or slide the spare wheel under the side sill as an emergency fail-safe. Only begin work once stability has been verified. Under no circumstances should anyone crawl beneath a vehicle supported only by a jack.

Operating procedure: When using a hydraulic jack, position it squarely on level ground, tighten the pressure release valve fully, align the saddle securely with the vehicle jacking point, and pump the lever handle. Once work is completed, slowly turn the release valve counter-clockwise to allow a smooth, controlled descent.

Scissor jack usage: Before winding, verify that the notched groove on top of the scissor arm engages firmly with the vehicle's pinch weld seam. Never stomp on or kick the jack linkage to force it down, as this risks bending the mechanism or causing the car to slip off.

Maintenance

Routine maintenance directly affects a hydraulic jack's operational safety and service life. Hydraulic fluid acts as the working medium; over time, it degrades and picks up contaminants. It is recommended to flush and replace the hydraulic oil every six to twelve months to maintain smooth operation and internal lubrication. For long-term storage, keep the jack in a dry environment with the ram fully retracted and wiped clean. Before each use, inspect the housing for fluid seepage and check that the oil level is adequate—any fluid leak can cause the jack to suddenly drop while under load.

Relationship with Regulations and Periodic Vehicle Inspection

While a car jack itself is not an item tested during periodic vehicle roadworthiness inspections, its condition and usage can impact inspection outcomes. Standards such as the "Technical Specifications for Safety of Power-driven Vehicles Operating on Roads" (GB 7258-2017) require vehicles to carry standard emergency on-board tools. During visual safety checks, inspectors ensure standard safety items like warning triangles are present; while jacks are rarely mandatory fail-items, missing on-board kits may be flagged for rectification. Furthermore, during undercarriage structural inspections, any damage caused by improper jacking points (such as a crushed oil sump or deformed chassis rail) can lead to a direct inspection failure.

Technical Development Trends

With the growing adoption of new energy vehicles (NEVs), jacking technology has seen corresponding upgrades. Airbag jacks utilise compressed air or exhaust gases to inflate heavy-duty rubber bags; their broad contact footprint distributes pressure evenly without damaging fragile underbody battery enclosures or side skirts, making them ideal for low-slung sports cars and battery electric vehicles (BEVs). In workshop environments, electric hydraulic jacks are also growing in popularity—operating via 12V vehicle power or onboard batteries to lift vehicles at the press of a button, drastically reducing manual effort. Nevertheless, neither airbag nor electric jacks can replace mechanical axle stands, which remain mandatory before carrying out any under-car operations.