Key Slot Standards
- . The slots for the incoming mail must be at minimum of 1.75 inches high by 10 inches wide. Slots on locking mailboxes must be able to hold the residents daily volume of mail. The slots must be large enough to accommodate any unfolded USPS Priority Envelopes.
- What is the 'standard' method for dimensioning slots? We have for years located the slot by the geometric centerlines and described the slot size by note. For example: 'SLOT 9/16 X 1'. Some of us are wanting to locate the slot with the longitudinal centerline in one direction and by the ends of the slot in the opposite direction.
- WOODRUFF KEY SLOTS AND KEYWAYS J503195905 Revision History Related Info. TUBING, FLARED, STANDARD DIMENSIONS FOR, DESIGN STANDARD.
These have a cross slot about 5/32” wide. 1913 (MIL-STD-1913 “Picatinny”) AND 1913 The 1913 key is wider than the standard key because the slots are nominally.206” wide. (A little more than 3/16”) The standard key will work with a Picatinny rail but the light will move back and forth with recoil unless the clamp is very tight. Contact Internal Tool Location. 1813 Yeager Ave La Verne, CA 91750.
Keys are usually made on the shop floor by mechanics in accordance with published standards. However, this simplified approach is likely to produce keys made of the wrong material and in the wrong shape. The reason: published standards only cover key and keyway dimensions. Most standards fail to discuss the important issue of key material, as well as their shape and installation.
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Key material
To avoid key failures due to overload, choose a key material with the same strength and hardness as the shaft or hub. The reason for this can be found by examining the equations for calculating key stresses.
As shown by the last equation in the box, key stress equals shaft stress when the key (or hub) length is 1.6 times the shaft diameter. But modern couplings, particularly those made from alloy steels, have shorter hubs than this. They are usually equal to, or only slightly longer than the shaft diameter. In this case, key stress is about 50% higher than shaft stress. This would appear to require that the key material be 50% stronger than that of the hub; however, because part of the torque is transmitted through friction between the shaft and hub bore, the key material need only be as strong as the shaft material. Key stock is available from most steel suppliers in various grades of steel, including alloy steel.
Key geometry and fit
An improperly fitted key can cause costly maintenance problems, or even machine failures. Before installing a hub for a drive component, ensure that the key has the correct shape and dimensions. Then make sure that the key fits properly as follows:
• Tight in the shaft keyway.
• Sliding (not clearance) fit in the hub keyway.
• Clearance fit radially — a small clearance between top of the key and bottom of the hub keyway.
• Key length extends inward from the shaft end (never beyond) to beyond the hub end, by at least the rounded portion of the key.
The first two of these conditions ensure that the hub can’t rotate, even slightly, on the shaft. If a hub rotates (slips) on its shaft, every torque reversal causes hammering on the sides of the key, leading to damage. As damage to these contact surfaces occurs, clearance develops between them, and the key will eventually shear. A sliding fit between the key and hub keyway is recommended for ease of assembly and disassembly and can cause a hub to split. To ensure that the hub seats properly on its shaft, provide some clearance between the key and the bottom of the hub keyway. In the case of a coupling hub, this clearance provides an opening through which water or corrosive gases can enter the coupling, causing damage to its internal surfaces. To seal this opening against contaminants, apply a room temperature vulcanizing (RTV) sealant on top of the key before installing the hub.
The corners of a key must be chamfered so they do not interfere with the fillet radii in the keyway, Figure 2. On the other hand, too-large a chamfer reduces the area of contact between a key and the sides of the keyway. Under load, and particularly during shock loads, these smaller contact surfaces are more easily damaged.
A loose-fitting key, Figure 3, allows forces generated by torque to roll the key, causing high edge loadings between the key and keyway. Such edge loading can shear the key, Figure 4.
Tapered keys (plain or Gib-head) are sometimes used instead of setscrews to hold hubs from sliding on their shafts. Driven in too far, they cause poor hub-toshaft contact, Figure 1; if not driven far enough, they allow sliding between hub and shaft. Unfortunately, there is no way to check if tapered keys are properly installed. For these reasons, you should avoid using tapered keys for coupling applications.
Design, manufacturing, and installation errors
A key with a square end is likely to damage the shaft when transmitting torque, Figure 5. The corner of the key creates a high contact stress in the shaft, which can cause it to fail. To avoid such failures, use only keys with rounded ends.
The transmission of torque through a key must occur over the full length of the hub, otherwise twisting movements can occur between the hub and shaft. These movements cause shaft fretting (surface damage caused by small alternating motions), which leads to fatigue failure. If a key has a round end, the only way to transmit torque over the full length of the hub is to extend the key beyond the hub, by at least its rounded portion. Figure 6 shows the wrong approach: although the end of the key is rounded (as it should be), the key transmits torque only over its straight portion.
The relationship between key length and hub length also affects system balance. For example, a void between the rounded end of the key and the end of the hub keyway, Figure 6, causes the system to be out of balance. This condition can cause vibration especially at the high speeds typically encountered in electrical motor operation.
On the other hand, a key that extends beyond the hub also causes imbalance. If a balanced coupling is required, use a notched key, Figure 7. As a rule-ofthumb, a coupling should be balanced if it is going to be installed on the shaft of a motor with a balanced rotor.
The most often encountered error in key manufacturing is the use of key steel that is softer than the shaft. Remember that the key material and hardness should be similar to that of the shaft or hub.
This article is based on the book “Flexible Couplings: their design, selection and use,” by Michael Calistrat.
Michael M. Calistrat is a consultant on power transmission design and failure analysis and owner of Michael Calistrat & Associates, Missouri City, Texas.
This information applies to the American Steel Industry.
Holes and Slots
Standard Holes
Standard hole sizes for bolts are made 1/16-in. larger in diameter than the nominal size of the fastener body. This provides a certain amount of play in the holes, which compensates for small misalignment’s in hole location or assembly, and aids in the shop and field entry of fasteners. In the absence of approval by the engineer for use of other hole types, standard holes shall be used in high strength bolted connections.
Key Slot Standards Meaning
Although most holes for high-strength bolts are made 1/16-in. larger in diameter than the bolt body, certain conditions encountered in field erection require greater adjustment than this clearance can provide and approval from the engineer is required.
The maximum sizes of holes for bolts are given in the table below, except that larger holes, required for tolerance on location of anchor bolts in concrete foundations, are permitted in column base details.
Standard holes shall be provided in member-to-member connections, unless oversized, short-slotted or long-slotted holes in bolted connections are approved by the designer. Finger shims up to 1/4-in. may be introduced into slip-critical connections designed on the basis of standard holes without reducing the allowable shear stress of the fastener.
Oversize and Slotted Holes
When approved by the engineer, oversize, short slotted holes or long slotted holes may be used subject to the following joint detail requirements:
(1) Oversize holes may be used in any or all plies of connections in which the allowable slip resistance of the connection is greater than the applied load. Oversized holes shall not be used in bearing-type connections. Hardened washers shall be installed over oversized holes in an outer ply.
(2) Short slotted holes may be used in any or all plies of connections designed on the basis of allowable stress on the fasteners provided the load is applied approximately normal (between 80 and 100 degrees) to the axis of the slot. Short slotted holes may be used without regard for the direction of applied load in any or all plies of connections in which the allowable slip resistance is greater than the applied force. Washers shall be installed over short-slotted holes in an outer ply; when high strength bolts are used, such washers shall be hardened.
(3) Long slotted holes may be used in one of the connected parts at any individual faying surface in connections designed on the basis of allowable stress on the fasteners provided the load is applied approximately normal (between 80 and 100 degrees) to the axis of the slot. Long slotted holes may be used in one of the connected parts at any individual faying surface without regard for the direction of applied load on connections in which the allowable slip resistance is greater than the applied force. Where long-slotted holes are used in an outer ply, plate washers or a continuous bar with standard holes, having a size sufficient to completely cover the slot after installation, shall be provided. In high-strength bolted connections, such plate washers or continuous bars shall not be less than 5/16-in. thick and shall be of structural grade material, but not be hardened. If hardened washers are required for use of high-strength bolts, the hardened washers shall be placed over the outer surface of the plate washer or bar.
(4) Fully inserted finger shims between the faying surfaces of load transmitting elements of connections are not to be considered a long slot element of a connection.
Nominal Hole Dimensions
Anchor Bolt Holes
Hole sizes for steel-to-steel structural connections are not the same as hole sizes for steel-to-concrete anchorage applications. In the case of steel-to-steel connections, the parts are made in a shop under good quality control, so standard holes (bolt diameter plus 1/16″), oversized holes (bolt diameter plus 3/16″), and short and long-slotted holes can be used quite successfully. However, the field placement of anchorage devices has long been subject to more permissive tolerances (and often, inaccuracies that exceed those tolerances anyway and may require consideration by the structural Engineer of Record).
AISC published Steel Design Guide Series 1, Column Base Plates back in the early 1990s. At that time, it was recognized that the quality of foundation work was getting worse and worse. To allow the erector (and designer) greater latitude when possible, the permissible hole sizes in base plates were increased. These same larger hole sizes were included in the 2nd ed. LRFD Manual. The values there are maximums, not a required size. Smaller holes can be used if desired. Plate washers are generally required with these holes because ASTM F436 washers can collapse into the larger-sized holes, even under erection loads.
The larger hole sizes are primarily intended for the majority of base plates that transfer only axial compression from the column into the foundation. The anchor rods don’t usually do much after erection in that case.
To allow for misplaced bolts, holes in base plates are oversized. The AISC Manual of Steel Construction recommends the following oversized hole diameters for each bolt diameter:
Key Slot Standards Test
Anchor Bolt Hole Dimensions
Key Slot Standards
AISC, Steel Design Guide Series 1, Column Base Plates, suggests that using oversize holes meeting these criteria may still not accommodate field variations in anchor bolt placement and suggests adding 1/4 in. to the hole diameter listed. The guide recommends using a heavy plate washer over the holes. The AISC Structural Steel Educational Council cites the following example: “If bolts are misplaced up to 1/2 inch, the oversized base plate holes normally allow the base plate and column to be placed near or on the column line. If the bolts are misplaced by more than 1/2 inch, then corrective work is required.”
Keys For Old Slot Machines
Based on AISC oversize holes, the AISC Structural Steel Educational Council recommendations, and concrete contractor anchor-bolt placement techniques, ASCC (American Society of Concrete Contractors) concrete contractors recommend the following tolerance for each bolt location:
- 3/4-in. and 7/8-in. diameter bolts: ±1/4 in.
- 1-in., 1-1/4-in., and 1-1/2-in. diameter bolts: ±3/8 in.
- 1-3/4-in., 2-in., and 2-1/2-in. diameter bolts: ±1/2 in.
End of article.