Question

Differences in means

1. Does quench water temperature affect average yield strength

2. Does O-ring hardness affect average leak pressure

3. Is the average pressure capacity of an air tank above spec

Differences in variances

1. Does quench water temperature affect variation in yield strength

2. Do harder O-rings reduce variability in leak pressure

3. Is the variation in pressure capacity of an air tank within spec

Answer 1

1) Assuming the material to be steel;

Water quenching is probably the oldest heat treatment process used by man to harden steel. It can be described as the rapid cooling of metal from the solution treating temperature, usually in the range of 845oC to 870oC for steels. Quenching is usually performed in order to prevent ferrite or pearlite precipitation and facilitate the formation of martensite or bainite. In the hardened condition, steel should have 100% martensite to attain maximum yield strength. The severity of water makes the hardened steels brittle and in some cases developed internal cracks thus limiting their engineering application. These shortcomings of water quenched steels have almost made the phrase ‘water quenched and tempered’ a form of heat treatment since all water quenched steel is tempered. With tempering, the properties of quench steel could be modified to decrease hardness and increase ductility and impact strength.

2) Failure pattern: Occurring primarily in dynamic seals involving reciprocating, oscillating, or rotary motion, this failure pattern can be identified by a flattened surface on one side of the O-ring's cross section.

Problem sources: Metal surfaces of the gland are too rough and act as an abrasive . . . Metal surfaces are too smooth causing inadequate lubrication, Poor lubrication, Excessive temperatures . . . System fluid contaminated with abrasive particles.

Use recommended metal finishes. Provide adequate lubrication (consider internally lubricated O-rings). Check material compatibility with system temperature. Eliminate abrasive contamination with filters and/or wiper seals. Consider changing to a more abrasion-resistant O-ring material, such as carboxylated nitrile or urethane.

3)Capacity is the amount of air that the compressor can pump out. It is expressed in CFM (cubic feet per minute).

Now don’t be fooled by compressor manufacturers who state the size of the air tank (“a 200-litre compressor”). I see this a lot, especially for the smaller home/workshop compressors. Tank size doesn’t say a thing. It’s only a buffer between the compressor and the user.

The capacity of the compressor is in ‘liters per minute’, not in ‘liters’. You can fit a very small compressor on a 1000 litre tank, but it will take ages to fill up the tank.

So, how much capacity do you need?

Add up the requirements for all the air tools and machines that you have.

This is is the maximum capacity that your equipment requires together. That is, if you use all the tools and machines at the same time (which usually is not the case).

If you already have one or more compressors and need to buy a replacement machine, it is advisable to do a flow measurement to find out the maximum needed capacity.