Tutorial - Rack ’˜em and stack ’˜em

In a follow up to our recent examination of thermal management issues, this month’s tutorial focuses on the other design considerations for AV equipment racks – structural and cable management issues.

When undertaking equipment rack design and construction, there are a few basic steps to follow. The first is to establish the kind of rack you are going to need. This decision will be based on the purpose of the rack, the amount of kit you need to fit in it, and the location the rack is to be sited. In the case of restricted access you might opt for a pull-out / rotate type, and if space is really tight, it’s worth considering a narrow solution.

When selecting rack height, it’s always worth allowing 3-4U more than the equipment needs, or in the case of really big racks, 25% redundancy is a good rule of thumb.

Once you’re into the build phase, safety of yourself and co-workers should be the primary consideration, along with taking care of the equipment itself. When installing equipment, consider laying the rack on its back for particularly heavy items, thus reducing the amount of lifting required. Start installation from the bottom of the rack, working upwards and to maintain a low centre of gravity in the finished build, you should aim to site half of the total weight in the bottom third of the rack. A low centre of gravity stabilises the rack and reduces the risk of it toppling during build or once installed.

At this point you should also be taking into account thermal considerations and where you plan to locate any vents or fans. (See InAVate November 2007, or www.inavateonthenet.net for further tips on thermal management in racks.)

Careful thought should also be given to the ergonomics of positioning displays, input devices or source equipment. Don’t put a DVD player, control panel or monitor at the bottom of the rack! No one will be able to comfortably reach it.

Whilst physically installing equipment it is important to take note of the so-called cantilever effect. Using the correct order for mounting shelves and equipment will prevent damage to the equipment and reduce the risk of accidents during installation. Attach both lower screws to the equipment, before attaching the top ones so that the weight of the equipment is supported by the faceplate, rather than pushing it out. (See fig. 1)

Saving time

Integrate in the workshop, install in the field is a mantra you should follow. Pre-building your racks means you have all the tools to hand, you can trouble shoot before arriving on site and problem solving is much simpler to carry out – faulty connections or even equipment are much easier to replace back at base than in the field. It also provides a much more secure working environment.

Wherever possible, lay the rack down. Whilst this is most important for heavy items, it generally makes life easier when screwing into the rack, as well as making things safer from a stability view point. If your rack becomes fully loaded, gang racks together side-to-side. Don’t attempt to expand upwards. Finally, wherever possible plan for future expansion by leaving space, and spare power slots. This will make things much easier a year or so down the line if your client needs to upgrade or add new equipment.

Cover the rear

Rear support of equipment is another important consideration when dealing with heavier items. If any of the following is true, then you should use rear support: The unit weighs more than 4.5kg per U, the centre of gravity is towards the rear of the unit, the depth of the unit is greater than 2.5x the racking height.

There are several rear-support options. You can use a solid lacing bar for additional support at the back. Middle Atlantic supply rear handing brackets as well as zero height shelves and adjustable rack shelves, all of which provide additional rear support. For a temporary solution you can use wooden blocks.

Cable concerns

Cable management is the other vital piece of the rack building puzzle. Following best practice when bundling, supporting and running cables delivers important benefits:

 It improves system reliability.
 It leads to faster installation times.
 It results in racks that are easier to service in the long term.

At all times you should: Avoid running signal and power/speaker cables parallel and ensure signal and power cables cross at right angles. These steps will minimise signal interference. Proper cable management maintains the correct bend radius for cables to protect them from damage.

Lacing bars should be used to reduce the strain on both cables and connectors, improving system life and reliability. They also provide support for horizontally run cables. When using lacer bars to provide strain relief they should be positioned close to the rear of the unit (See fig. 2). Multiple lacer bars can be used at different depth positions to manage bundles of vertical cables.

Cable management on a rotating-type rack can lead to additional challenges. Firstly, decide in which direction you want the rack to rotate. Dress the cable horizontally on a rear cable management rail, keeping power and signal cables on opposite sides. Pull out and rotate the rack to determine the required length of cable, and then dress the cables on to the opposite side of the frame from the equipment.

Points to remember:

 Always maintain a gap between power and signal cables.
 Power and signal cables should only ever cross at right angles.
 Offset bars allow you to get in close to the back of the unit without adding additional rails.
 Leave room for cable labels.
 Prior testing on a bench allows better planning of your cable runs.
 When mounting lacer bars, always use short screws to protect cables from chafing.

Following the above advice should put you well on the way to producing an easily-maintained and good looking rack. Whilst aesthetics are not necessarily the primary concern, a well put together rack demonstrates to your client that you take pride in your work, as well as making it easier to find your way around in case of problems.

More info

This article was prepared with kind assistance from David Clark of RGB Communications in the UK. More information on the products they distribute can be found at www.rgbcomms.co.uk

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