Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2000-07-19
2002-12-17
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06494103
ABSTRACT:
FIELD OF INVENTION
This invention relates to load creation apparatus, and to a method of testing the ability of a load bearing structure to bear a test mass that is created using such a load creation apparatus. The invention relates particularly, but not exclusively, to loads which are used for testing the load bearing characteristics of hoists, cranes, davits, winches and other load bearing structures. The invention also relates to the testing of static load bearing structures, such as foundation piles, bridges, floors, jetties and wharves.
Specifically, the invention does not relate or extend to containers and apparatus in general that are not adapted or designed for load creation and testing purposes.
BACKGROUND
The ability of a load bearing structure to bear maximum loads will decrease with its length of service, due to wear and tear of the apparatus. For this reason, it is necessary and sometimes mandatory that load bearing apparatus, such as hoists, cranes, davits and winches be tested periodically to verify that the apparatus are still capable of functioning at the maximum load capacity. In addition, such tests may be required when substantial alteration or repairs are made to the apparatus.
The tests require the load bearing structure or apparatus to successfully bear a specified test load or, more precisely, a test mass. Typically, test masses are in the range of a few metric tonnes to more than a few thousand metric tonnes and, in most cases, are in the range of 25% in excess of the safe working load of the load bearing structure or apparatus. Earlier test masses have been made of concrete or metal. Over a period of time, such test masses made of concrete can chip or crack and may thus loose their original precision of mass due to wear and tear. Metal objects will tend to rust, and may in some cases suffer from metal fatigue or crack propagation. Furthermore, these tests are conducted only periodically. It is appreciated that between tests, the storage of these solid test masses can create an inconvenient storage problem. Furthermore, the problem is exacerbated when a range of test masses, required for different apparatus, must be stored.
In response to the problems inherent in the use of solid test masses, the use of flexible liquid-filled bags as test masses has been proposed in United Kingdom Patent No. 2,047,414B (Tonnes Force Testing Services Limited) and United Kingdom Patent No. 2,072,351 (Water Weights Limited). The bags used in these British patents consist of flexible envelopes that are filled typically with water in order to create the necessary test mass. The liquid-filled bags resemble generally pear-shaped objects that are supported either singly or in combinations to create the specified test mass. Since these flexible bags are collapsible when empty, they solve the problem of storage that is inherent in the use of metal or concrete test masses. However, while one set of problems is overcome, the use of water or liquid weights introduces a different set of problems that is inherent in the use of temporary masses, namely the problem of ensuring that the bag is filled with the correct amount of liquid to create a mass of the correct value.
This problem of filling the bag accurately with the correct amount of liquid must be seen in the light of the fact that the standards of weight testing often require the masses to be within relatively fine tolerances. For example, reference is made to British Standard BS 7121 Part 2: 1991 which is entitled “Code of practice for safe use of cranes. Part 2. Inspection, testing and examination”. According to British Standard BS 7121, the test mass should be of proven accuracy to within ±1.0%.
Another factor that presents difficulties in creating a temporary mass within the acceptable margins is that the density of a liquid varies with composition and temperature. Consequently, changes in the liquid density affect the mass. A cubic meter of pure water at 40° C. weighs around 7.7 kg less than another cubic meter of water at 3.98° C. The density of pure water at 40° C. is around 992.3 kg m
−3
, whereas at 3.98° C. it is around 1000 kg m
−3
. The density of sea water taken from the open sea may vary between approproximately 1020 and 1030 kg m
−3
. When expressed as the amount of salt per kilogram of sea-water, in terms of parts per thousand by weight (o/oo), the salinity of sea-water varies typically from 34 to 37 o/oo and has been found to be as low as 5 o/oo in the Finland and as much as 41 o/oo in the northern part of the Red Sea. The difference of density in sea water is caused mainly by the variation in temperature and salinity. Fresh water and sea water are the most common liquids used to fill the weight testing bags. Hence, if a bag is filled consistently with the same amount of liquid, the vagaries of liquid density means that the bag may not necessarily contain the same mass on each occasion. These variables mean that the amount of liquid required to fill the bag is not constant, and must be ascertained in the context of the ambient conditions. These factors explain why the task of filling bags, with what can amount to thousands of liters of liquid, is not simple task, especially when the amount of liquid may have to be within ±1.0% of a specified amount as required by certain mandatory standards.
The vagaries discussed above mean that it is often necessary to weigh the mass using a weighing device such as a load cell or dynamometer. Although weighing the mass, at the time of using the test mass, can ensure that the vagaries of the amount of liquid and the prevailing density of the liquid are taken into account, the use of a weighing device, in turn, presents another set of problems. It is appreciated that the calibration (and indeed the maintenance of that calibration) of weighing devices used for masses in the range of a few thousand tonnes is expansive and not straightforward. Furthermore, the precision of the calibration is lost progressively over time, which means that the expensive process of calibration must be repeated regularly. For example, in British Standard BS 7121, the weighing device or weighbridge used to ascertain the value of the mass must, at the time of conducting the test, have been calibrated and certified within the last twelve months.
In the field of weight testing, the weighing devices usually include electrical components and circuitry. The use of such electrical weighing devices presents peculiar problems in applications where inflammable materials are in close proximity. For example, when testing cranes that are used in petroleum production facilities, it is necessary to ensure that the electrical weighing devices are shielded, so that any sparks from the weighing device will not initiate ignition of the petroleum products in the vicinity.
Another type of load bearing structure which must be tested with a test load is found in the piles used as a foundation of buildings. A test method is defined in A.S.T.M. D 1143-8 (American Society for Testing and Materials) which is entitled “Standard Test Method for Piles Under Static Axial Compressive Load.” After the foundation piles have been driven into the ground, the piles must be subjected to a static compressive load to test whether each pile has adequate load bearing capacity. The test loads are usually created by soil, rock, concrete, steel or water-filled tanks. These solid masses are usually solid, and are often in the form of large concrete blocks. Therefore, problems similar to the ones mentioned above in connection with solid test masses are experienced in transporting and positioning the large, solid masses on the test rig, and in storing the solid test masses when not in use. Furthermore, a weighing device in the form of a hydraulic jack is required to determine the actual test load, and this hydraulic jack must be regularly calibrated and certified. Other types of load bearing structures that must be tested with compressive loads include bridges, floors, jetties and wharves.
An object of the present in
Allen Andre
Fuller Benjamin R.
Welsh & Katz Ltd.
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