A common questions is asked - " How do I determine the load my crate can tolerate?"7
"Testing Crates." Wood Crate Design Manual. Vol. 252. Dept Of Agriculture. 73-74. Print.
The following is an excerpt from the manual. Please refer to the full article for each testing method information on each factor that determines load tolerances.
There can be no absolute standards or rules for determining crate load capacity. For example, while a crate top may be designed for loads up to 200 pounds per square foot, it might be subjected to a load of twice this limit under difficult storage conditions. However, because of the safety factors included in the design, short-time application of such static loads does not ordinarily cause failure. Nevertheless, it is reasonable to assume that some crates will be subjected to loads that will cause failure. Existing published standards for crates do not provide load capacities but instead provide sets of information that the designer must review and apply those that are based on the specific situation for that crate.
Of principal concern are the fundamental principles of design and the relationships of various details in the construction of containers that are balanced in strength. Special testing machines and methods of testing have been developed. From this research, supplemented by study and observation of shipping containers in service, has come much information of value to packaging engineers.
Notable among the findings and developments of the Forest Products Laboratory is the evolution of crate design criteria for virtually any type of machine or other industrial product. These criteria are based on the following considerations:
1. A crate must be strong enough to protect its contents from the hazards of shipping and storage.
2. The lumber and other materials used to build the crate must be of suitable quality and dimensions.
3. A crate must be as light in weight as shipping hazards and the inherent strength properties of the materials permit.
4. It must require a minimum of shipping space.
With design criteria based on these considerations, the effective engineering of crates for specific purposes becomes possible. This handbook presents information of a general nature applicable to the design of most types of crates and the solution of crating problems. It is not intended to be a specification; however, in order to clarify design and construction of crates, a number of crate designs are included to aid the designer in his specific problem. It includes all data required for the design of crates, such as allowable working stresses for the various species of wood and the method of determining fastening requirements
But designing all crates to resist. every stress is not economical. Thus, both design standards and tests to check them should be based on conditions to which the majority of crates are subjected. The design of a crate may vary greatly for any item, depending on the means of transportation and the destination.
FACTORS THAT AFFECT CRATE DESIGN The selection of a crate depends on, in general order of importance: These factors overlap.
3. method of transit
4. handling hazards
5. storage conditions
Domestic shipment of most items will not ordinarily involve as great or as many hazards as shipment overseas. Consequently, in domestic crates frame members may be smaller and fastenings less rigid. But some means should be used to evaluate a newly designed pilot crate, whether for domestic or export shipment. Tests should simulate actual shipping, handling, and storage conditions as closely as possible in order to determine the suitability of construction.
The development of crate design criteria at the Forest Products Laboratory led to a series of tests designed to include those hazards ordinarily imposed on crates during storage, handling, and shipping. These tests were used on scores of containers of all sizes and types, from small open to large sheathed crates. The following sections outline the various test methods that might be used by the designer to improve crates or to verify construction details. However, with a crate design based on engineering principles, a limited crate testing program may be satisfactory. A good indication of the crate ís adequacy could be gained from superimposed load, edgewise-drop and cornerwise-drop tests.