Mechanically Generated Friction Fits in Power Transmission Couplings 

In the world of electric motors and power transmission coupling products, torque is increasingly being transferred between directly coupled shafts by means of purely mechanically generated friction rather than by positive drive connections like keys and keyways or splines. Mechanical friction fits evolved into modern standardized dimensions during the twentieth century as machine tools and cutting tooling evolved into their present states of precision and accuracy. For many decades power transmission component design lagged behind the machining world when it came to the use of mechanically generated frictional clamping, with a large portion of connecting elements still relying on keyways and interference fits to guarantee transmission.

Interference fits, wherein the diameter of the male component is actually slightly larger than the diameter of the mating female component, do have their merits when it comes to ensuring reliable backlash free transmission.  But they can be difficult to assemble, as the shaft must be cooled with a cryogenic such as liquid nitrogen, or the bore needs to be expanded with heat. If either high heat or extreme cold are used, safety is a key concern during the assembly process. Heating a metal component will also often change its mechanical properties such as temper, so cooling the male fitting part is preferred when maintaining the material properties is a concern. Due to the inconveniences associated with interference fits, most newly designed frictional clamping coupling hubs are made using compression by means of screw thread fasteners instead. 

This type of mechanical connection is very predictable and can be made by applying a set amount of torque to radial or axial screws. For easy assembly the fit between the male and female part typically should not have overlapping diametrical tolerances, but should also not allow for excessive clearance. When the fits can be made by hand, proper tightening of the fasteners is all that is necessary to create a friction fit which will reliably and predictably transmit torque for the life of the machine.  A shrink disk style fit on a female member of a coupling will maintain concentricity and will compensate for tolerances on a male shaft while maintaining the ability to transmit torque across a range of diameter dimensions and torque values. This type of conical clamping hub can be manufactured in different styles to promote modularity, ease of assembly and concentricity between shafts, often allowing for easier balancing for high speed applications. 

The fastener assisted friction fit has streamlined design of couplings as it allows for the use of smooth shafts which are not subject the imbalance and complications shaft key or spline tolerances can cause. The next time you are sizing and selecting rotating components, consider going with keyless frictional fits and reap the benefits of modern machine construction.

Balanced High Speed Precision Couplings

As a manufacturer of precision couplings, we spend quite a bit of time on high speed applications. Here are a few basic tips about coupling balance.

A properly balanced drive component should not induce excessive vibration into the drive line. This means that the mass of the component rotating about the axis of the shaft must be evenly distributed in such a manner as to maintain an even centrifugal force about the axis of rotation. The magnitude of the centrifugal force generated by a rotating component varies with the location of the center of mass of the drive component versus the axis of rotation and orientation of the moment of inertia. 

A simple way to think of static balance about an axis is to imagine playing on a see-saw in a playground.  Two people of equal mass can sit at an even distance apart from each other and maintain balance. Or one person of a heavy mass can sit close to the pivot point and a person of a light mass can sit far away from the pivot point and maintain balance. A rotating mass works essentially the same way. As an object with constant mass moves away from the axis of rotation, the centrifugal force increases exponentially. In physics, this is simply illustrated with a moment of inertia calculation (I=mr^2).

In practical manufacturing, rotating parts are made with evenly spaced jaws, spokes, etc.  Balancing comes into play when manufacturing processes cause imbalance or additional parts such as clamping screws or shaft keys are added. Adding or subtracting mass in one area of a part causes an imbalance. This must be corrected by adding or subtracting mass on the opposite side of the part. Manufacturers most often do this by drilling holes into the part near the outside diameter so that less material will need to be removed resulting in a more structurally sound part versus removing a lot of material near the axis of rotation.

Precision parts such as machined coupling hubs are designed with the balancing holes in a location so they can be produced in quantity in a numerically controlled process without balancing each part individually. This provides cost savings and consistency to customers. Quality small to medium sized machined hubs should come with a speed rating of about 10,000 RPM. Many of these hubs can be finely balanced for speeds between 30-80,000 RPM depending on size. The larger in diameter and more massive a hub becomes, the lower the practical balance speed will be.

Other considerations such as torque transfer at high speed need to be examined as well. Centrifugal forces at high speeds will begin to open clamping hubs for example. Since the screws of clamping hubs are normally tightened to a torque which takes them near their tensile yield, the effects of centrifugal forces applying additional screw tension as the clamp opens must be considered. This is why in medium to large sized hubs, conical clamping rings are often favored, since they offer better symmetry and better stress distribution as a result of the forces applied to them at high speed.

R+W makes a lot of standard and custom couplings for high rotational speeds, some of which rated to handle speeds in excess of 150,000 rpm.

The coupling shown at left is one of our miniature designs. Higher torque versions are available on request.  Contact us with your high speed coupling requirements and get the ultimate connection.

Mounting a Sprocket or Pulley to a Torque Limiter

As a manufacturer of torque limiters for indirect drives, we are often asked to provide some assistance when it comes to mounting the torque limiter inside a drive sprocket, pulley, or gear.  R+W does offer to provide torque limiters as a complete package, with the drive attachment pre-mounted according to customer specifications, but we are also happy to provide customers with some guidance when it comes to doing this on their own, including providing machining drawings for a do-it-yourself or third party project.  In this article we’ll focus on mounting a roller chain or timing belt sprocket to a torque limiter, as they represent the vast majority of requirements for indirect drive torque limiters.

The process is really straight forward, and can usually be performed in any machine shop with a milling machine and a lathe.  Once the correct torque limiter body size has been selected, based on the required bore diameter and disengagement torque, it must be compared dimensionally with the size of sprocket being used, in order to determine whether it can be mounted directly to the torque limiter, or if it will need to be offset mounted. While a drive sprocket with a smaller diameter than the torque limiter output flange can be offset mounted on a separate bearing, and attached using an adapter plate as needed, it is usually easier and less expensive to choose a sprocket that will fit directly.

Most quality torque limiters include a bearing (1) between the base of the clutch and the output flange.  This helps to ensure that the driving and driven portions of the torque limiter are properly guided within the rotational axis after disengagement.  In order to protect the bearing from moment loading, the belt or chain tension (2) needs to be well centered over the bearing, unless it will be supported by an external bearing, as is the case in offset mounted systems.  Centering the tension over the bearing in the torque limiter often requires that an additional pocket be machined into the drive attachment, so that the torque limiter can be sunk into drive attachment to a depth that places the bearing underneath the drive medium (i.e. belt or chain).  R+W offers an allowable load centering range, in terms of a distance from the end face of the torque limiter.  This load centering range, dimension “S” from the R+W safety couplings catalog, is the range in which the center of the chain or belt must reside for smooth, sound operation. 

Once the dimensions have been selected, machining the mounting features into the sprocket or pulley is fairly straight-forward. First an inside pilot diameter is bored into the sprocket on a lathe. This bore should be precise to match the centering diameter on the torque limiter output flange.  The centering diameter is referred to as dimension “E” in the R+W safety couplings catalog, and it is the contact surface which ensures that the sprocket will be well centered around the drive axis.  An ISO H7 tolerance is recommended for the pilot diameter, which normally runs between half a thousandth to just under two thousandths of an inch oversized from the nominal diameter, depending on the size, with the larger tolerances applying to larger diameters.  The pilot bore should also be concentric to the pitch diameter of the pulley or sprocket. Lathe jaws with a flat grip for the tips of the sprocket teeth and a machined recess to hold the face plane of the sprocket perpendicular to the axis of rotation allow for a simplified setup. 

Next, any relief needed for centering the chain or belt tension over the bearings in the torque limiter is turned into the sprocket’s face.  This diameter must be greater than dimension “G” from the R+W safety couplings catalog, and is less critical than the pilot bore diameter, since it is only for clearance.  It is important however that the sprocket or pulley is square in the lathe chuck, since the turned face in the resulting pocket needs to rest flat against the face of the torque limiter output flange in order for the drive to run smoothly.

Finally, the clearance hole bolt circle is drilled into the sprocket on a milling machine or drill press. The size and number of holes can be taken from dimension “H” in the catalog, and the bolt circle diameter taken from dimension “F” in the catalog.  Using a rotary table makes this process quick and easy. These holes can be counter-sunk or counter-bored to save axial space if installation space is at a premium. 

Once the sprocket or pulley is finished, it is ready to be mounted to the torque limiter output flange.  Inspect the machined surfaces to ensure that they are clean and free of nicks, burrs, and debris.  Slide the drive attachment over the centering pilot on the torque limiter and rotate, while applying gentle pressure, to ensure a proper fit.  Little to no relative movement between the drive sprocket or pulley and the torque limiter output flange should be possible, aside from rotation.  Insert the mounting screws and ensure that they are finger-tight.  Evenly tighten the screws in a crosswise pattern, applying 1/3, 2/3, and finally 3/3 of the recommended tightening torque for the size and type of screw being used.  

CALCULATING MOMENT OF INERTIA

As a manufacturer of safety couplings and torque limiters, we are often asked to provide some assistance in calculating the moment of inertia of various loads, in order to aid in selecting the appropriate disengagement torque settings.  Once the driving and driven inertia values are known, acceleratin and deceleration rates are used to estimate safety coupling torque adjustment ranges. 

Moment of inertia can be described as a mechanical property of the mass of a solid object that quantifies the required torque needed to change the angular velocity of the object about an axis. This inertia is also known as the mass moment of inertia, first moment, or, rotational inertia.  This moment can be used to calculate how much energy it takes to get an object rotating which is especially useful in mechanical drive lines with motors and engines.  When sizing a torque limiter or safety coupling, it is important that the disengagement torque value be set to a value higher than what is required to accelerate the load up to speed.  It is also helpful to be aware of the torque levels resulting from abruptly decelerating a rotating mass, which is where torque limiters and safety couplings come into play, protecting driveline components from the excessive torques resulting from an unintended blockage stopping a machine too rapidly.  Moment of inertia often denoted as (I) is a unit of mass multiplied by area squared (M*A²). In the case of a two inch diameter rotating shaft with a mass of one hundred pounds, moment of inertia can be calculated per the example below.  

Calculating the moment of inertia for a 0.1 meter diameter shaft with a mass of 1000 kg is done in the exact same manner.

Finding the moment of inertia for a tube or hollow shaft is very similar. In the next example, we will find the value for a 0.5 meter outside diameter tube with a 0.3 meter inside diameter. The mass of the tube is 500 kg. 

The calculation for a moment of a square or rectangular shaft or shape can also be useful. In this example, the moment of a one inch by one inch square shaft with a mass of 1000 pounds will be found.

The examples above are handy for calculations involving sizing of mechanical drive components, and not just safety couplings. Inertia ratios of rotating shafts and motor rotors are critical to proper operation of many drives. These calculations are all done about the axis of typical shaft rotation and cannot be used for a different axis.  It is important to note that this is the mass moment and the calculation gives us a property of mass at a distance from the axis of rotation. As with all rotating objects, the moment increases exponentially as a mass moves away from the axis of rotation.  These basic facts of physics are critical for machine design and have been a primary guiding principle for centuries of mechanical engineering.

Proper Shaft Fits for Precision Drive Components

When sizing high performance drive components, proper shaft fit is an important consideration in helping to maintain concentricity and balance in the rotating equipment. In v-belt drives, chain drives and other low-cost / low-speed systems, designers often give little thought to shaft fit. Many manufacturers of these types of low cost drive components are simply relying on a shaft key and set screw to transmit torque and retain the hub on the shaft, and other types of misalignment absorbing elements help to take up any eccentricity resulting from a relatively large clearance between the shaft and the bore.  But when it comes to high-performance / high-speed drives and “smart” motors, such as servo and steppers, shaft fit becomes increasingly important.

When researching a possible vendor for zero backlash components with engineered clamping systems that can be assembled by hand, it is imperative that they are able to provide a shaft tolerance as well as a published chart which verifies the shaft and bore fits required. As a general rule of thumb, the overall diametrical clearance between the shaft and the bore should be around 10-50 µm (.0004-.0019”) for smaller diameter shafts, and as shaft diameters increase beyond 80mm (3.150”), larger clearances become allowable, and more annulus is required around the ID of the bore to ensure easy installation. If the fit is correct, a bit of oil is all that is required to easily slide a hub onto a shaft. 

 Many engineers are inclined to specify interference fits for high performance drives, which in a solid hub design would require either sub-zero cooling of the shaft and / or, more commonly, heating of the bored and keyed hub. This type of fit dates back to times before frictional clamping systems became well established in the power transmission industry, and is intended to eliminate problems associated with loose shaft fits and backlash.  Requiring this type of work can create a maintenance and installation problem, and is not necessary in most modern applications incorporating engineered clamping systems. But the importance of precision shaft fit remains when it comes to frictional clamping style hubs. 

Frictional Clamping Hubs

In the case of clamping collarstyle hubs, which offer the advantage of very easy installation and removal, if the fit clearance is too large between the shaft and bore, once clamped, the hub can become eccentric to the shaft.  This can cause an imbalance and or misalignment issue in addition to the potential for failure of the driveline. In self centering clamping systems, often used for high speed / high power systems, excessive fit clearance can reduce the transmittable torque of the shaft hub connection.  But with proper shaft fits and screw tightening torques applied, they offer superior performance to simple keyway hubs, and far easier handling than interference fit hubs. 

 (clamping collar)        (self-centering clamping system)

Manufacturing Standards

There are several standardized shaft tolerance charts commonly used in industry such as ANSI B4.1-1967 (R2004) for imperial units and ISO tolerance system per DIN 7160 (8.65) for metric units.  Precision drive components should have a clearance fit which is just out of range of a transition fit such as an H7/h6 fit per DIN 7160 (8.65) chart. For example on a 32 mm diameter shaft, this would mean that the shaft could be 0-16 µm under 32mm.  The bore could be 0-25 µm over 32 mm. This would result in a diameter difference of 0-41 µm (.0000-.0016”).

If the shaft and bore were both the same size, assembly would be difficult. In practical manufacturing, this would almost never happen. A good machinist would normally use a H7 go/no-go plug gauge and an h6 go/no-go ring gauge. The go portion of the plug gauge would not practically fit into the bore by hand unless the bore was slightly larger than 12 mm exactly. Vice-versa, with the ring gauge and shaft, when checking a turned part with a ring gauge, it would not fit by hand unless the shaft was a bit smaller. If either no-go plug or ring gauge fits, the parts are scrapped. This method of machining/checking shafts and hubs results in parts with a precise fit which can be assembled by hand. 

For more information on proper shaft fits and precision couplings for high performance drive applications, contact applications@rw-america.com.

MTBF APPROACHING INFINITE

The concept of fatigue resistance in flexible shaft coupling design has been highly valued by R+W since its inception in 1990.  For much of its first two decades in business the company focus was almost exclusively on couplings for high performance servo drive technology.  When it comes to machinery that utilizes this type of equipment, professionals at all levels know that shut down for maintenance can be extremely costly, and that unplanned downtime can have catastrophic effects on the profitability of a process.  When properly applied, the flexible bellows coupling addresses this and a great number of other concerns in support of high speed, high accuracy machinery.  In addition to fatigue resistance it offers the benefits of high torsional stiffness, low moment of inertia, and continuous symmetry, all of which lend themselves very well to motion systems involving rapid indexing and high precision positioning – essentially making it the first choice for servo drives.  For many years R+W has been well known as a leader in bellows coupling technology.  But as the company has continued to grow and add couplings for higher powered industrial drives to its portfolio, the need for a different type of fatigue resistant metallic coupling has become apparent.  In 2013 R+W is introducing its SURVIVOR series of flexible disc pack couplings.  Not to be confused with a servo coupling, the flexible disc pack coupling is ideally suited for many of the most demanding industrial power transmission systems made.