Jacks

Types of Jacks

Jacks are widely used in industry where loads need to be raised or lowered a
limited distance.

They are often used to afford access for other means of handling or lifting
equipment and for levelling purposes. Their versatility of use and portability make
them ideal for many maintenance functions. It is due to this that, in the past, they
have generally been incorrectly considered to be a tool rather than a lifting
appliance.

Jacks are either hydraulically, pneumatic or mechanically motivated by manual

or power systems. The range of designs, sizes and capacities is almost limitless,
and many manufacturers produce as standard jacks designed for a specific
function, e.g. re-railing locomotives.

The Screw Jack

A screw jack is a mechanical device that can increase the magnitude of an effort force.

The jack comprises a hollow body, usually cast iron or cast steel, with a female thread at the top, into
which a screw fits.

The top of the screw has a large head which is cross drilled and a tommy bar is fitted through this to
enable the screw to be turned directly by the operator. A large acme or square thread form is used as
this is strong in compression and resists stripping. It also offers a large 'land' area so that the effects
of wear are minimised.

As the efficiency is less than 50% the screw remains in the position it is
placed, whether under load or not.
Another common type of screw jack is the ‘trailer jack’ which is used as parking jacks on mobile
gantries.

The Journal Jack

The journal jack also utilises a screw thread form to gain lift and obtain the load sustaining feature
necessary.

A ratchet handle is used to turn a pair of bevel gears, which in turn cause the screw to rotate. The
jacking column, or lifting journal, has a female thread in its base into which the screw is fitted, thus
providing the lifting/lowering motion.

The thread form used in this case is a buttress thread. Buttress threads are designed to resist heavy
axial loads in one direction making them ideal for jacking operations.

This arrangement gives fast, but very controlled, lifting so enabling precise load positioning and
levelling.

Click on the hotspots below:

The Ratchet Jack
The ratchet jack utilises the lever and fulcrum principle to raise and lower the load, via a rack and
pawl mechanism. However, it requires great physical effort to lift the rated capacity although lowering
the load is somewhat easier. For this reason they are often referred to as lowering jacks.

They have the advantage of faster operation than screw jacks.

A toe on the lifting rack enables them to be placed under much lower items than with screw jacks,
although the load that can be taken on the toe is only 25% of that that may be taken on the head of the
jack.

Click on the hotspots below:

The operating lever is linked to a short pawl, the lifting pawl, that engages with the rack and lifts it a
single tooth at a time. A long pawl, holding pawl, also engages with the rack. This holds the rack in
position sustaining the load whilst the lifting pawl is moving to another tooth.

When the mechanism is placed in the downward position, effort on the lever allows the short pawl to
carry the load while the long pawl is kept out of the way. As the rack approaches the lower limit of
each lever operation the mechanism pulls the long pawl forward to engage in the teeth of the rack.
This in turn holds the load whilst the short pawl travels upward to take the next bite.

Click on the hotspots below:

Trailer Jacks
Another common type of screw jack is the ‘trailer jack’ which is used as parking jacks on mobile
gantries.
Hydraulic Jacks
Hydraulic jacks use oil as the lifting medium, the body of the jack acting as a reservoir for the oil.
When the jack is operated, oil is passed under pressure through a system of non-return valves to the
base of the ram chamber. As more oil is delivered with each stroke of the operating lever, and as the
oil cannot be compressed, it forces the ram out of the chamber providing a lifting motion. Lowering is
achieved by releasing a valve which allows the oil to return to the reservoir. The operation of the
release valve may be by depressing a lever, turning a screw or opening the valve directly. The ram is
then returned by gravity, although in some cases this may be spring assisted.

The controlled use of hydraulics, where known pressures are applied over known areas, enables
jacks to be used for load weighing purposes. Calibrated gauges may be fitted to some models to
enable them to be used for this purpose.

In the hydraulic jack we make the oil flow into a confined space under pressure, but as we cannot
compress the oil, the space must expand. If the space cannot expand, the weakest part of the jack
would fail, e.g. the seals would blow. The space is expanded as a result of the oil pushing the ram out
of the jack. In the illustration, we see a schematic sketch of a jack showing the principle of operation.
(An independent pump and ram would be the same except the pump is connected to the ram by a
hose with the delivery non-return valve in the entry port to the ram).
Hydraulic Jack Bodies are commonly manufactured from aluminium, steel or cast iron. The
material used affects the design, size, self-weight and capacity of the jack.

Claw Attachment
The use of a claw attachment provides the means of supporting the load on the head of the jack or on
a toe.

It should however be noted that the maximum load permitted on the toe of the claw attachment is
considerably less than on the head, usually only 40% of the head load.

Furthermore, it should also be noted that only jacks with extended bases can be used in conjunction
with claw attachments as these prevent the jack from overturning.

Click on the hotspots below:

In this case the lifting ram has a robust male thread along its length and a screwed collar is fitted to
this.

This enables a raised ram to be locked in position against the base of the jack.

Should there then be any loss of pressure within the hydraulic system, the load will be supported by
the collar against the base, preventing the ram from creeping down.
Click on the + below

Basic Operation of a Hydraulic Jack:

In hydraulic pump and rams, or jacks, the liquid is a light oil. Generally this flows easily, offers
considerable resistance to compression and is not greatly affected by changes of temperature. So,
although not strictly true, we can consider the oil to:

• Be incompressible, so that when put under pressure it will flow

• Unaffected by temperature so that it will not expand
• If put under pressure will seek an increase in the volume of the space
In the hydraulic jack we make the oil flow into a confined space under pressure, but as we cannot
compress the oil, the space must expand. If the space cannot expand, the weakest part of the jack
would fail, e.g. the seals would blow. The space is expanded as a result of the oil pushing the ram out
of the jack. In the illustration we see a schematic sketch of a jack showing the principle of operation.
(An independent pump and ram would be the same except the pump is connected to the ram by a
hose with the delivery non return valve in the entry port to the ram.)

When the lever is raised the small (pump) plunger is lifted. This causes oil to be sucked from the
reservoir into the area previously occupied by the plunger. The oil passes through a filter and a non-
return valve (suction valve). When the lever is then depressed the plunger is lowered putting pressure
on the oil, which, due to the non-return valve is unable to return to the reservoir. The oil is therefore
made to flow into the large plunger chamber via another non return valve (delivery valve).

Further pumping of the lever causes more oil to flow under pressure and, as the pressure acts equally
in the enclosed system, it seeks to expand the space available to it. This it does by pushing the large
plunger (ram) out of its chamber, the pressure in the system being greater than the downward
pressure of the large plunger and load.

To lower the jack a valve is opened (release valve) which allows the oil to return to the reservoir. The
downward action of the ram puts pressure on the oil, which, unable to return the way it came
because of the non-return valves, flows back to the reservoir by this alternative route.

If the load were too great the jack would fail due to the high internal pressure. In practice a pressure
release valve is fitted so that if the pressure builds to an amount greater than for which the valve is set
the oil is allowed to flow back to the reservoir.
See below an example of a Multiple Jacking Operation from One Power Pack.

Critical components

Typical components to be included during a thorough examination.

• Jack body
• Marking
• Operating Lever
• Ram and Load bearing component
• Jacking mechanism
Examination points, assessment, and rejection criteria
The jack must regularly be thoroughly examined by a Competent Person to check whether it remains safe to use.
This is to be done within a maximum period of 12 months unless a written scheme of examination (for guidance refer
to LEEA 032 Guidance to Written Schemes of Examination), drawn up by a competent person is in place and
operating.

Reports of thorough examination should be compliant with the legal requirements or the LEEA template report
documents, retained and cross referenced to the trolleys historical records for inspection by the Competent Person
or the enforcement authority.

Any defects found by the examination should be reported to the owner of the equipment, who must assess the root
cause of the defect and implement procedures to prevent reoccurrence, e.g. training of operators, increased
inspections, etc., before remedying the equipment and returning it to service.
The competent person may deem it necessary to supplement their examination with testing. Such testing could be
NDT, creep test, functional testing, overload testing, etc. The nature and extent of testing is always at the discretion
of the competent person in support of their thorough examination.

Unless a mandatory requirement of the applicable national legislation or manufacturer, LEEA does not recommend
the routine overload testing of trolleys, except following an exceptional circumstance such as significant
modification or repair. This is because overload testing has few benefits and number of disadvantages:

• Some manufacturers do not recommend overload tests, except in ‘exceptional’ circumstances

• Repeated overloads can cause deterioration of the equipment over time
• Most structural failures are the result of fatigue and such defects will not be revealed by an overload test;
fatigue cracking can be identified during thorough examination
• Defects such as fatigue cracking can be made worse by overload testing but may still not be identified by the
test
• If equipment fails during testing it could be dangerous and will certainly be expensive
• Inspection bodies do not recommend it as there is no defined structural or mechanical benefit

Defined Scope of Examination

For each of the components checked in the defined scope and relevant
supplementary testing the lifting equipment examiner will have a predefined list
of acceptance and rejection criteria. The following is a non-exhaustive list of
such criteria:

Click on the + below

Marking

• The presence, accuracy and condition of markings, such as SWL, identification marks, year of
manufacture for example

Jack body

• Corrosion, cracks, damage, distortion, wear affecting the strength or functionality

• Security of attachment points and sub-structures, fasteners, welds, etc.

Operating lever

• Corrosion, cracks, damage, distortion, wear affecting the strength or functionality

• Full or partial seizure of rotating components
• Adequacy of lubrication
• Lever does not move back under load greater than that specified by the manufacturer

Load ram and bearing component

• Corrosion, cracks, damage, distortion and wear affecting the strength or functionality
• Adequacy of lubrication
• Security of attachment points
 • Creep under maximum load within manufacturers tolerances

Jack mechanism, including brake, ratchet, gears, pinions and bearings

• Alignment within the manufacturers tolerances

• Corrosion, cracks, damage, distortion, wear affecting the strength or functionality
• Adequacy of lubrication
• Mode of operation as intended by the manufacturer
• Full or partial seizure of rotating components
• Creep under maximum load within manufacturers tolerances
• Oil patches or obvious leaks, particularly during the operation of the jack. This is usually due to
failed seals or 'O' rings but may also be a sign of cracked jack body
• The jack fails to lift or after a short period of movement the load rises and falls with each stroke
of the lever. When operation ceases the load begins to lower on its own. These may be due to
blown seals or non-return valves not operating correctly and can be caused by dirty/emulsified
oil, or due to scored, nicked or otherwise damage to the ram face
• Ram fails to lower, fully retract or lowering is jerky - usually due to dirty/emulsified oil causing
valves to stick, but may be due to other damage. Similarly, the release valve may be difficult to
operate or fail to release the load
Action arising from a thorough examination
• A record of the examination should always be made at the time of the examination irrespective
of whether the equipment passes or fails or whether the user immediately withdraws it from
service
o This record can be on the job paperwork, in the examiner's notebook, on a portable
computer or by any other suitable means provided that it is authenticated by the
examiner
o If the information is subsequently transcribed for presentation to the user, the original
record made contemporary with the examination should be retained as evidence of the
examination
• If the equipment is found to be defective or requiring repair within a short time, a responsible
representative of the user should be informed as soon as practicable
• If the defect is such that it presents an immediate or imminent danger, the user should be
notified immediately and advised to withdraw the equipment from service without delay. In
some regions, there is also a statutory duty on the examiner to send a copy of the examination
report to the enforcing authority
Repairs
When repairs are made, only components which are the correct size, material, grade and, if appropriate, approved
by the manufacturer should be used. The original equipment manufacturer's instructions should be followed. A
record should be made of all repairs. The examiner should enquire about any repairs made which might affect the
safety of the equipment.

Repair using certified components

Where certified spare parts are used, e.g. a replacement bottom hook, a subsequent proof load test is not usually
necessary and is at the discretion of the examiner. After repair, the equipment should be examined to ensure that
the work has been carried out correctly. The certificate for the replacement item should be filed with the record for
the equipment.
Repair using uncertified components or affecting the SWL
After a repair using uncertified components, or where the repair or process involved may affect the safe operation
and use of the equipment, the equipment should be proof load tested and then thoroughly examined.