Difference Between Hardness and Strength in Steel

One of the most common points of confusion among our clients has been the difference between hardness and strength in steel. You wouldn’t believe the number of product designers who often fail to differentiate between hardness and strength. The two are confused more often than not.

So what is the actual difference between the two?

Strength is defined as the ability to withstand an applied load without failure. Hardness, on the other hand, is defined as the ability to resist deformation. But even though the two are different, they are also directly related. Increase one and the other follows suit.

Simple enough, right? You’d think so, but the amount of people who mix them up would suggest otherwise. It is important you be able to understand the respective definitions of each term, as it could be important in the development of your own products.

Interested in talking more about hardness, strength and other important terms? We’d love to chat with you! Contact us at ProvenProductivity@bossard.com to get the conversation started.

For more shopping options click here.

A Vital Part of the Metric System

In our previous blog post, we covered the tolerance system. By using the ISO tolerance system, any combination is possible to get the needed fit. In order to limit the number of tools as well as the number of machine elements, ISO created the base hole/base shaft system.

In the base hole system, the hole is given a fixed tolerance. Since it is easier to modify the diameter of the shaft, the base hole system should be given preference. Most metric reamers produce a hole tolerance of H7. This results in less required tools.

In the base shaft system, the shaft is given a fixed tolerance in order to achieve the required fit.

Here are some standard combinations of fits:

Snug Fit H7/s6
This is used in high pressure bolted joints. The tolerance of the shaft is larger than the hole tolerance and exceeds the zero line.

Sliding Fit H7/h6
This type of fit is used to fit parts that can be assembled and disassembled freely. The shaft tolerance is slightly under the hole tolerance.

Drive Fit H7/m6
For this type of fit, exact positioning is needed, usually in a shear load. The shaft tolerance is much larger than the hole tolerance and the shaft has to be driven into position by force.

Loose Running Fit H7/d9
In this combination, the shaft tolerance is much smaller than the hole tolerance with ample clearance. Used for bearing in off road machinery, farm equipment, and where there are large temperature variations.

Have more questions about tolerance systems or the different combinations of fits? We have the answers! Contact us at ProvenProductivity@bossard.com to start the conversation.

A Vital Part of the Metric System

In a previous blog post, we wrote at length about the benefits of metric fasteners – and the use of the metric system, in general. The importance of tolerance systems was mentioned, but not expanded upon. The ISO tolerance system is, in essence, the key to global interchangeability.

The key to understanding the metric system as it relates to fasteners is the understanding of its tolerance system. The inch/pound system tolerance is done by minimums and maximums that are determined by design.

The ISO fits and tolerance system is based on a number/letter system that relates to the ISO tolerance chart. This chart is applied globally and allows the designed parts to be interchangeable.

The letters indicate the location of the tolerance and the numbers indicate the tolerance range. The bigger the number, the bigger the spread between the maximum and minimum tolerance. Capital letters show internal dimensions (holes, sockets key, etc.) while lower case letters refer to external dimensions (shaft diameters, pin diameters, width across the flats, etc.)

Each letter has its exact location related to a so-called zero line. The zero line refers to the nominal dimensions such as nominal diameter and nominal length.

In our next blog post, we’ll cover fits as it relates to tolerances, so be sure to check back next week! Until then, if you’re interested in engaging further about tolerance systems, please do not hesitate to reach out! Contact us at ProvenProductivity@bossard.com to get the conversation started.

The use of incorrect washers happens more than you might think – and it occurs all across the globe. What are the consequences of using the wrong washer? The strength of the join is reduced and the risk of loosening in a dynamic joint is increased.

So how do you know which washer to use?

Comparing and contrasting lock washers, split lock washers and ribbed lock washers can help create a better understanding of the advantages and disadvantages of both types.

Flat washers are up first. When used correctly, flat washers can help reduce surface pressure on softer materials, minimizing the loss of clamp load due to embedment. You’ll notice that the bearing area is often larger than a screw or a nut. In addition to producing more frictional resistance, flat washers also protect softer materials and reduce the risk of loosening due to embedment.

You’ll need to reference the fastener property class to determine the proper washer hardness. Choosing the wrong washer hardness can increase the risk of embedment.

Split lock washers are up next. There is a misconception that these are added to reduce the risk of rotational loosening, but the true objective with split lock washers is to reduce the loss of clamp load due to embedment. Correct use will reduce the risk of loosening due to dynamic forces.

But the strength of this type of washer is often overrated, leading to a much higher risk of loosening due to embedment. Split lock washers are only able to withstand clamp forces introduced by fasteners up to property class 5.8. It is also important to mention that their effectiveness is very low or even non-existent when used with heat treated fasteners of 8.8 (grade 5) and higher.

Ribbed lock washers stand apart. These washers feature ridges on at least one side designed to anchor themselves into the clamped parts, as well as into the bolt or nut’s bearing area. The friction in the bearing area prevents spontaneous rotational loosening of the screw or nut by increasing friction between bearing areas. This washer is also intended to reduce the risk of embedment.

For the reasons stated in the paragraph above, ribbed lock washers represent a viable and effective alternative to flat and split lock washers.

Not sold on the idea of ribbed lock washers for your application? We’re here to help! Contact us at ProvenProductivity@bossard.com to start the conversation.

Never underestimate the importance of preload. Failing to achieve the correct amount can have devastating and far-reaching consequences. Here, we’ll address two of the most common failures that can occur in the event of insufficient preload.

Mechanical overload failure – including embedment, crushing, yielding and fracturing – is nothing you ever want to deal with. But excessive preload can result in exactly that. Too much applied torque will lead to failure at assembly, and the consequences of that can have a domino effect. This will add to subsequent service loads and exceed the strength of the fastener/joint material.

Specific problem areas related to mechanical overload failure include differing thermal expansion rates, material, and creep especially when the joints contain plastic or gaskets.

Insufficient preload can lead to transverse slip of joint members, which in turn can cause loosening. Self-loosening, leaks, slippage, squeaks and rattles are all things you don’t want. Ensuring that the preload is sufficient will help avoid a lack of clamp force and the ensuing effects.

Obviously, the importance of preload cannot be ignored. Failing to implement the correct amount will certainly lead to issues and complications you’d rather not have to deal with.

Interested in learning more about the importance of preload and the potential consequences of failing to use a sufficient preload? Contact us at ProvenProductivity@bossard.com!

Metric vs Fractional Fasteners

Can’t decide between metric fasteners and fractional fasteners? Allow us to settle the debate.

There are several benefits to using metric fasteners, chief among them being that metric dimensions are globally interchangeable, enabling you to procure parts from anywhere in the world.

For this reason, metric fasteners help facilitate international communication and trade, in addition to lowering in place costs. Products made in the United States are easier to service and maintain and also become an easier sell overseas, due in large part to the interchangeability of products.

The metric system is also simple and logical. The advantages are myriad. In addition to the aforementioned interchangeability, the metric system also benefits from increased accuracy, and has been applied by almost all nations across the globe.

Metric fasteners are subject to the ISO tolerance system. This system system is the key to achieving lower cost and interchangeability globally.

For more information about the benefits of metric fasteners, click here.

The Bossard website also features a tool for converting between metric and inch fasteners. This converter – which can be found here – offers best proposals as you work toward a compromise.

Not sold on metric fasteners over fractional fasteners? The conversation doesn’t have to end here. Contact us at ProvenProductivity@bossard.com! We’re happy to continue this discussion with you.