In the late 1800s when common battery telephone systems were introduced the 19" rack was born. It's not clear if this was the exact standard we have now. Later in telephone service there was a need for wider racks but these maintained the hole spacing now used on 19" racks.
It may have been in the 1930s and 1940s that the current standard was established, see here.
The specification is EIA-310-C "Racks, panels, and associated Equipment".
There's also IEC 60297 Mechanics for Racks - 19 inch common standard (DIN41494}.
Early on HP recognized that many of it's customers were mounting their test equipment into 19" "relay racks". This is a practical way to build a test system for many reasons. If a test equipment manufacturer does not allow for this they are eliminating sales.
Almost all "bench" instruments are designed to also allow rack mounting. This may require removing the bench feet, carry handle, rubber bezel or other enclosure parts and the addition of a rack mount kit that typically contains the "ears" and other hardware.
The space between the two vertical rack mount attaching rails is 17 3/4". Any equipment that's rack mounted must be narrower than this. The horizontal distance across the "rack mount ears" on the equipment is 19". Instruments could be a fraction of a rack width. The HP/Agilent 4395A Spectrum/Network/Impedance Analyzer and the HP 54501A Digital Oscilloscope are 16 3/4" wide and are full width instruments. The HP/Agilent 34401A Multimeter is 8 1/4" wide and is 1/2 rack width. The HP 204 Audio Oscillator, HP 432 Power Meter and the 427A Voltmeter are all 5 1/8" wide or 1/3 rack instruments.
10-32 tapped holes 18.3" center to center on verticals.
There are mounting kits for all rack standard sizes of instruments
Rack Width Instrument width full 16 3/4 (< 17.75) 1/2 8 1/4 1/3 5 1/8 1/4 3 5/8 ?
The screw holes in the rack are not all on the same center to center spacing, but rather are repeating between 0.5", 0.625" and 0.625". This works with a set of standardized panel heights that are all multiples of 1 3/4" which is called 1 Unit high. All rack mount equipment is an integer multiple of 1 Unit. The location of the notches that pass the 10-32 mounting screws is standardized for each panel height. Note that the location of the notches is vertically symmetrical allowing the equipment to be installed right side up or upside down.
1/2"- 5/8" - 5/8" repeating. RU is from center of space in 1/2" gap
22 May 2008 - while trying to fit the FTS4060 into a rack it appeared that the notches were in the wrong place. But before cutting metal a through analysis of how panels can be notched followed.
There are two ways that the holes can be located in a panel. One way is to have the edge of the panel centered between two holes that are 1/2" apart. The other way is to locate the holes so the panel edge is centered between two holes 5/8" apart. Note that the sum of the three cyclic hole spacings is 1.75" i.e. the 1U pitch. Because of that if one edge of a panel that is an integer number of units high then the other edge will fall between holes of the same pitch. The half the distance between two holes 1/2" apart is 1/4" or 0.250 which is a nicer number than half of 5/8" i.e. 5/16 or 0.3125" most panels center the edge on the 1/2" holes.
If all the panels in a rack have been made on 1/2" or all on 5/8" edges then they will stack one panel right next to the one next to it. BUT, if you have two different edge spacings then there will be a wider gap between adjacent panels with different edge location layouts.
The holes are symmetrical top to bottom allowing for installing a panel upside down.
When installing a panel the bottom two holes carry virtually all of the torque that's trying to let the center of gravity of the instrument rotate downward. If you install the bottom two screws they will hold the instrument. But if you install only the top two screws the bottom edge of the instrument will tend to move away from the panel and if a heavy instrument may break free. The loading on the bottom screws is minimized if they are as far apart as possible. That's more important than having more screws.
Units Height inches Edge centered on 1/2"
Notches from top inches
Edge centered on 5/8"
Notches from top inches
1 1.75 1/4 1 1/2
5/16, 1 7/16
2 3.50 1/4 3 14
3 5.25 1 1/2 3 3/4
4 7.00 1 1/2 5 1/2
5 8.75 1 1/4 3 5 3/4 7 1/2 (4 holes) 1
1 3/4 4 3/8 5 (3 holes)
6 10.50 1.75, 3.34, 6.75, 8.75
etc. etc. etc.
Note 1 - corrected error 22 May 2008
Racks come in a number of heights from table top units to ones that are just a little too tall to go through some door you need to go through. When this happens it may be easier to take apart the door frame than it is to take apart the rack loaded with equipment. Some are just bare frames and others are very fancy.
Sub ModularHP made a number of instruments they called Sub Modular. They are all 6 3/32" high.
They can be combined into a rack size using the 5060-0797 Adapter Frame. When this is used it is not easy to add or remove instruments.
They can also be easily combined using the Model 1051A Combining Case. This case has flexible plastic retainers that can be easily opened to allow an instrument to be installed or removed. These retainers can also be moved horizontally to allow different combinations of instruments to be installed. It will hold three instruments that are 5 1/8" wide, which was a very popular size.
I'm not aware of any standards about depth.
Many of the HP instruments have a groove running left - right along the top and the front bench feet have a tongue that will fit into the groove of an instrument that was stacked below it. The groove ends about 1/4" before each side so when instruments are stacked they are interlocked both front to back and side to side.
Instruments that do not have this feature tend to slide off the instrument below them when you are connecting cables or otherwise pushing on them. They typically don't make it to the floor, but tend to stop after the feet are off the instrument below them.
The term Rack and Stack is used to describe the ability of combining instruments in an organized manner and that's why I chose it as the name for my business of building this type of system.
RailsMany racks built to hold electronic equipment also have a pair of vertical rack mounting rails in the back. The most common use of these is to install a couple of "L" section bars front to back to act as a shelf to hold heavy equipment as it is slid into and out of the rack. This makes is much easier to work with heavy boxes.
These is also a way to get equipment into a rack that does not have "ears" using a shelf, but the rack can not be shipped because the equipment is not attached to the rack. Also not good if in Earthquake country.
The rear vertical rack mount feature can be used to mount equipment from the back side. If there is a front panel on the equipment you typically want that facing forward, but if you want to have a place to mount a lot of connectors, then installing a panel on the back is great. Also AC power entry panels and fans.
Blank PanelsAre used to fill up empty space both for appearance and in systems where forced air cooling is used.
Outlet StripsMounted vertically at the rear of the rack provide a place to plug in all the equipment. Two may be required if there's a lot of equipment in the rack.A comment on outlet strips in general. With older tube equipment the spacing of "Plug Mould" type outlet strips was many inches. These could be mounted to a work bench and provided maybe one outlet every 6 inches. This was OK when using tube equipment because of the 15 Amp limitation on a single AC circuit. With modern solid state equipment that draws very little current you need as many outlets as you can get on the strip.
On older strips the sockets are arranged with the flats aligned along the long axis of the strip. When a "wall wart" transformer or power supply is plugged into this type of strip it will cover one or two of the adjacent sockets. The latest power strips turn the sockets so the flats are at right angles to the long axis so that "wall warts" will go off to the side. They also space the sockets to allow for this, or maybe allow for one or two wall warts. You can also get short "Y" cables for connecting wall warts to outlet strips.
On-Off SwitchIt's convenient to have a single On-Off switch for the whole rack. That way you know the rack is turned off. Some instruments do not have lights so without careful inspection you don't know of they are on or off.
Power PanelWith older tube type equipment the power consumption of the total rack typically exceeded the 15 Amp limit for a single A.C. power cord so the rack might have a power panel that would take in 220 VAC, provide a main circuit breaker and feed the rest of the rack.
FansThese were popular with tube equipment and could be mounted at the bottom of the rack to blow the hot air out the top.
Custom Rack KitsThere are custom rack kits that are made to mount equipment that does not readily take rack ears. For example a receiver that was made to non standard dimensions or a computer monitor.
Keyboard SupportThere are special 1 unit high supports for computer keyboards
TableThis is a table that sticks out the front and can be used to support the unit under test, for a writing surface, etc. Some can be extended.
Reusable Pallet & Cardboard Box
Agilent has a standardized way to ship a rack. It is placed on a wooden pallet to allow a fork lift to move it. Inside there is a wooden ramp that's hinged so that the rack can be unloaded and reloaded onto the pallet. The cardboard side and top covers are reusable and are attached using reusable straps (as long as the straps are not cut).
Van with Power Lift
When I worked at Aertech HP brought one of the first automated Network Analyzer systems to our plant in Mountain View for a demonstration. They had modified a panel truck so that the rack was grabbed on the sides (about at the center of mass) and could be lifted and rotated to slide into the truck, much like a casket. This system used a 2116 HP computer with core memory. At lunch the sales rep loaded a football game where each player would pick a play: short run, long run, short pass, long pass or field goal. The probability of the play working was based on historical Cal vs. Stanford games, the field position, down and some randomness. Good for Monday morning quarterbacks.
Portable Crate-Rite Rack Mount Case
These provide a way to ship rack mount equipment that can be reused and take up much less space than cardboard boxes.
This is a shipping case for rack mount electronic equipment.
There's a little more than 14" (8 rack units) height available.
It came with two installed "L" brackets (at about the center) and four loose "L" brackets.
Also installed just below the "L" brackets are a pair of equipment slides, but not the mating parts for the slides.
It appears that the two installed "L" brackets were holding a piece of equipment between the bottom rails and the "L" brackets, thus taking a lot of the load off the equipment slides.
It sounds like a good idea to restrain equipment using the "L" brackets at the top and bottom so that all the load is not taken by front panel screws.
The equipment holes are not tapped and are oversize for a 10-32 screw. Clip-on metal nuts are installed where front panel mounting screws will be used.
I've seen a PRC-138 radio and it's associated power amplifier installed in a similar case, ready to use.
Marked Manufactured by
Environmental Container Systems
Clip for front mounting with 10-32 screws Tell me where to get
Bottom Foot that mates with cylindrical socket on top of another unit to allow stacking.
Spring backed 10-32 T-Nut to be used with "L" brackets
10-32 screws not included.
p/n: 47001440 Spring Backed T-Nut
p/n: 47001013 10-32 Nut with Clip
3482895 Protective Case for Electronic Instruments, Dennis M. Becklin, Crate-Rite Inc., Dec 9, 1969, 312/352; 206/305; 206/594; 220/4.02; 361/730
The inner equipment rack is connected to the shipping shell with shock mounts. Two or more cases can be stacked vertically using the locating cylinders.
That way the equipment can be used while mounted in the case just be removing the front and rear covers.
Rack 1 - although you can not see it in the photos two of these racks are side by side on a common base with wheels
Rack 2 -
Time & Frequency Rack -
When equipment is rack mounted in aircraft the ARINC 404 (Wiki) specification governs the system.
An example of an ARINC 404 ATR is the one for the Collins PN-101 Pictorial Navigation System, see Fig. 10
1/4 Short 2.25 12.62
1/4 Long 19.62 1/2 short 4.88 12.62
1/2 Long 19.62 3/4 Short 7.50 12.62 3/4 Long 19.62 1 Short 10.12
12.62 1 Long 19.62 1-1/2
PS ARINC seems to have a problem with their math.
The 3/4 size is exactly 3 times the 1/4 size. But the other ratios do not show integer relationships.
10.12 * 0.75 = 7.590 not 7.5
10.12 * 0.5 = 5.060 not 4.88
10.12 * 1.5 = 15.18 not 15.38
If you know why this is let me know.
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