This document covers a simple method to decide what size and shape of 80/20 t-slot bar is required for a particular project – based on the weight the structure must support. There are infinite ways of engineering a t-slot aluminum structure, but the method outlined below is simple and straightforward – and should be easy to understand by non-engineers. Although we reference 80/20 brand t-slot extrusion data in this article, you could draw usable conclusions for other t-slot aluminum profiles if their size, weight and shapes were similar to 80/20 Inc. t-slot products.
The basic principle of choosing the right bar is understanding when a bar starts bending (commonly referred to as deflection) under a specific weight load.
There are 9 common methods for calculating deflection for t-slot aluminum bars as outlined below:
Today, we will be looking at the Two-End Supported method using Load Centered weight (circled in the chart above). While the two-end fixed test method is closest to the common vertical corner post method of design, it assumes an unbreakable connection with the vertical post. As fastener strength will play a role in any fixed design strength, we will use the two end supported method to better illustrate independent bar strength. In general, bars will bend easier when supported on each end versus being actually fastened between two vertical posts; consequently, the data and conclusions provided here will have a magnitude of safety if used in a two end fixed configuration.
How strong is each extrusion type or bar? We get that question all the time. As outlined above, it all depends on what you do to the bar. But in our opinion, the cleanest, simplest way to judge bar strength and deflection is to look at how far a bar will bend if we put all the weight in the middle of the bar while supporting both ends.
At F&L Industrial Solutions, we use a target defection of 1/10” or under for most t-slot structures. If a bar has a chance of bending more than 1/10”, then we usually redesign with a stronger bar. So it is important to remember that target – a tenth of one inch or under.
Look at the chart below.
As you can see, the 80/20 t-slot called 1010 (which is a 1” x 1”) bar will bend 1/10” when 46 lbs. is applied to the center of a 36” long bar, while both ends are supported. If we were building a simple table, we might say that this table could hold a maximum of 184 lbs. (4 horizontal bars forming the table top) on the edges of the table (how much the center of a tabletop could hold would be dependent on the tabletop material and thickness – a good subject for another day). That same table designed with the 2020 series (2” x 2”) could hold up to 2,304 lbs on the edges.
Note: It is important to realize that some of the weaker fastening methods can only hold 200 – 400 lbs., so using a heavy-duty fastener is imperative to achieve the same strength as the bar. For example, there would be no reason to spend money on strong 2020 bars and match them with inexpensive and fairly weak corner brackets as the fasteners would break before the bar bent 1/10”. By using stronger connectors like end fasteners and anchor fasteners and external gussets, one can easily create connections that are as strong or stronger than the bar.
Click here to read about fastener strength in a past issue of Extrusion Builder News.
The graph above is a good starting point, but it is only one data point – for 36” long bars. A longer bar would bend at a much lower weight, and a shorter bar will bend a higher weight. Usually when we are designing a t-slot aluminum structure, we will add additional supports so that there is rarely long unsupported bars. For example, if a table top was 8’ feet long – we would normally put two additional cross braces under the table top, and at least 6 vertical legs – thereby eliminating any long bar from carrying too much of the load.
One last thing to think about when designing t-slot aluminum structures or frames with 80/20 or any other brand of aluminum extrusions is maximum load. While the cart or table or frame you are designing may only be used for holding a 50 lb. piece of equipment, what will happen if a large person stands on it and uses it as a way to change a high light bulb? When designing, you should design for the very worst case scenario that you want your frame to survive.
In summary, here are the 4 simple steps to determining the t-slot aluminum bar you need for your frame:
- What is the maximum load or weight that the structure will be exposed to?
- What is the longest bar length that will be used in your design?
- Find a deflection calculator for the t-slot aluminum bars you are using. An online deflection calculator for 80/20 is here.
- Using 1/10” or under as a target, what is the proper bar type to use at the length identified in Step 2.
Still confused? F&L Industrial Solutions offers free design and quotation of high quality 80/20 t-slot aluminum extrusion framing – so don’t be afraid to contact us to help. Click here to contact us to discuss your next project.
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If you would like to download and use the 80/20 Inc. deflection calculator that we used for this article, just click here to download.