Here, we detail what causes skidding in rotary systems. Then we analyze single and double-roller cam follower designs.
By Georg Bartosh • President | Intech Power-Core
For production machinery with moving subsystems, wear can be a major headache. That’s especially true for those featuring traditional metal cam followers, bearings, and other mechanical elements. As we’ll explore, polymer components can be a superior alternative because they avoid metal-to-metal wear as well as grease contamination of end products.
Consider skidding-related wear. Skidding occurs when a speed differential along contact surfaces causes metal components to slide against one another. This in turn accelerates wear-type equipment damage and can ultimately lead to unanticipated machine downtime. For many manufacturers (especially those that produce end products in bulk) even small interruptions can have big consequences — including missed shipping dates and lost customers.
With traditional metal cam followers, the solution to skidding is heavy lubrication of contact surfaces — sometimes to the point of over-lubrication. But this creates its own challenges — including contamination caused by stray lubricant, for example. What’s more, external lubrication systems are costly, labor-intensive to maintain, and non-sustainable.
In contrast, cam followers featuring advanced polymer elements are intrinsically self-lubricating to avoid these pitfalls. Plus, cam followers with polymer elements lend themselves to new and innovative mechanical designs.
Example application: A packaging-machine turntable
Consider a packaging machine that features a large 70-in. diameter turntable to transport cardboard packages between two points. Cam followers support the rotating part of the turntable. The cam followers are mounted on a lower stationary table surface constructed of flat steel plates.
The design called for 1.75-in. diameter cam followers carrying an average load of 380 lb but lubrication was not an option. So, the manufacturer selected lubrication-free 1.75-in. metal-equivalent polymeric cam followers.
At first, the manufacturer was satisfied with the turntable system’s output. But over time, friction-related temperature increases caused wear damage. More specifically, the cam followers’ surfaces became rough. The polymer slowly expanded, and the radial component of the friction force caused the outer racer to slip off the bearings. A new solution had to be found.
What was the problem?
Skidding caused the damage. It occurred because the inner and outer diameters of the cam follower ran on different diameters of the turntable. This generated heat, spurred wear, and shortened the cam followers’ lifespan. Consider the figure titled Skidding zone detail. For every revolution of the turntable, Points 1 and two on the cam follower had to travel different distances. At Point 1, it traveled 193.55 in., while at Point 2, it had to travel 200.15 in. to reach the same angular position as Point 1. Because the linear velocity of all the points on the cam roller is the same, smooth turning caused the roller at Point two to travel only 193.55 in. for every turntable revolution. Skidding compensated for the rest of the distance.
Eventually, the skidding — and the damage it caused — rendered the entire system inoperable, needing the manufacturer to order new parts and driving cost and valuable time.
Based on the calculations, the skidding distance between Points 1 and 2 is equal to 6.6 in. Given the inner extreme of the cam roller width at Point 1, the skidding distance increases as we move toward the outer extreme at Point 2 as shown in the Skidding zone detail figure.
In addition, the turntable rotates for thousands of revolutions — exponentially increasing the overall skidding distance. Such skidding leads to wear and tear due to friction, shortening the roller’s service life.
Twin cam-follower design benefits
Because lubricating the turntable wasn’t an option, the manufacturer needed an alternative solution to prevent the cam followers from skidding. That’s when the twin-roller design was identified as a leading suitable solution. This cam follower, which is more efficient and long-lasting, incorporates two rollers mounted on the same shaft — each one fitted with a precision ball bearing and running independently on the turntable.
To further minimize wear, each roller also has a slight crown. This reduces the roller-turntable contact from a line of contact to a point of contact. Because both rollers can run independently with different linear velocities, they can cover different distances via pure rolling, eliminating friction resistance and skidding altogether.
As shown in calculations, all points on a single-roller cam follower must travel at the same constant speed, causing the roller to skid 6.6 in. to cover the needed distance. Replacing this design with the twin cam followers enables both rollers to rotate independently with different velocities. As a result, they can travel different distances to cover the same angle turned by the turntable.
In this way, since their implementation, the twin cam followers have successfully eliminated skidding in this turntable system — without any lubrication. This has more than doubled the life of the cam followers compared to that of the single-roller design.
In addition to packaging applications, this twin cam follower design is suitable for any rotary application.
Increasing the number of the single rollers — to three, for example — will also increase the load-carrying capacity.
In-depth analysis of the issue
Now let’s calculate the amount of skidding with a standard single-roller cam follower. What follows is a quantitative analysis of cam-follower skidding in our example turntable application. Here, the single-roller cam follower remains stationary as the turntable rotates. As mentioned, because of contamination concerns, applying lubrication to reduce wear between the cam follower and turntable surface wasn’t an option.
These calculations involve a stationary cylindrical roller in contact with a circular steel plate that turns with a speed of 101 rpm. First, we can analyze the rolling behavior of the extreme points of the roller — Point 1 and Point 2 as shown in the image titled Single-roller skidding. Because the roller is flat, it makes a line of contact with the table. Points 1 and 2 denote the line’s two extremes. All the points along the line have equal linear velocities; this region between the two points is where skidding occurs.
Point 1 has a turntable diameter of 61.67 in. with the speed of the roller equal to 3,560 rpm. The roller diameter is 1.75 in. Our goal is to calculate the amount of skidding that occurs in this system, where:
The turntable diameter at Point 1 (in.) = dt1
The turntable diameter at Point 2 (in.) = dt2
The speed at which the turntable rotates (rpm) = nt
Toller diameter (in.) = dr
Turntable linear velocity (m/sec) = Vt
Number of revolutions the roller would travel at Point 1 per turntable revolution = Nr1
Number of revolutions the roller would travel at Point two per turntable revolution = Nr2
Number of turntable revolutions = Nt
Roller linear velocity (m/sec) = Vr
Roller angular velocity at Point 1 (rad/sec) = w1
Speed at which the roller rotates at Point 1 (rpm) = nr
Distance the roller travels at Point 1 per turntable revolution (in.) = L1
Distance the roller travels at Point 2 per turntable revolution (in.) = L2
Skidding distance of the roller per turntable revolution (in.) = Ls
Considering the single roller at Point 1
To find the distance traveled by the roller at Point 1 for one revolution of the turntable, we must find the velocity of the roller at Point 1. For this we can use the following formula:
Vr1 = ( dr / two ) × w1 = ( dr / two ) × ( two × π × nr )
Vr1 = 8.287 m/sec
To calculate the number of revolutions made by the roller in one revolution of the turntable, we can use:
Nr1 × dr = Nt × dt1
Nr1 = 35.239 revolutions ≈ 35.2 revolutions
Next, we can calculate the distance traveled by the roller in one revolution of the turntable, using the following formula:
L1 = ( Nr1 ) · ( π · dr )
L1 = 193.55 in.
The number of revolutions made at all contact points of the roller is the same, but the distance traveled at Point 2 of the table surface is different. Thus, we can calculate the distance the roller would travel at Point 2 for one revolution of the turntable if it were to rotate freely on the diameter (dt2).
Considering the single roller at Point 2
Using the same processes and formulas as above, we can find the distance traveled by the roller at Point 2 for one revolution of the turntable by calculating its linear velocity, along with the number of revolutions the roller would make for one revolution of the turntable. After doing so, we obtain the values:
Nr2 = 36.38 revolutions ≈ 36.4 revolutions
L2 = 200.15 in.
Because the distance values are different for both points on the roller, we can conclude that skidding takes place between Points 1 and 2 to compensate.
Calculating the turntable skidding distance
To find the skidding distance for one revolution of the turntable at Point 2, we can use the following equation:
Ls = L2 — L1
Ls = 200.15 in. — 193.55 in.
Ls = 6.6 in.
Thus, the roller skids for 6.6 in. between Points 1 and 2 for every revolution of the turntable.
Now let’s perform an analysis on the alternative twin cam-follower solution. For this quantitative analysis:
Roller diameter (in.) = dr
Turntable diameter at roller contact Point 1 (in.) = dt1
Turntable diameter at roller contact Point 2 (in.) = dt2
Speed at which the turntable rotates (rpm) = nt
Turntable’s linear velocity (m/sec) = Vt
Number of roller one revolutions per turntable revolution = Nr1
Number of roller two revolutions per turntable revolution = Nr2
Number of turntable revolutions = Nt
Roller one’s linear velocity (m/sec) = Vr1
Roller two’s linear velocity (m/sec) = Vr2
Roller one’s angular velocity (rad/sec) = w1
Roller two’s angular velocity (rad/sec) = w2
The speed at which roller one rotates (rpm) = n1
The speed at which roller two rotates (rpm) = n2
The distance roller one travels per turntable revolution (in.) = L1
The distance roller two travels per turntable revolution (in.) = L2
For these calculations, we can replace the single roller in with two rollers — roller one and roller two — of the same diameter (dr = 1.750 in.) turning adjacent to each other with different angular speeds and numbers of revolution depending on their distance from the turntable axis. Again, this is illustrated in the image titled Single-roller skidding. Because the rollers are crowned, we will calculate the velocity of each roller using contact diameter at Point 1 (dt1) and Point 2 (dt2).
Considering the travel of roller one
To find the distance roller one travels for one revolution of the turntable, we must find the velocity of roller one: we can use the following formula:
Vr1 = ( dr / 2 ) × w1 = ( dr / 2 ) × ( 2 × π × n1 )
Vr1 = 8.315 m/sec
To calculate the number of revolutions made by roller one in a single revolution of the turntable, we can use the formula:
Nr1 × dr = Nt × dt1
Nr1 = 35.417 revolutions ≈ 35.4 revolutions
Next, we can calculate the distance traveled by roller one in a single revolution of the turntable, using the formula:
L1 = ( Nr1 ) × ( π · dr )
L1 = 194.523 in.
Considering the travel of roller two
Using the same processes and formulas as above, we can find the distance roller two travels for one revolution of the turntable by calculating the linear velocity of roller two, along with its number of revolutions per revolution of the turntable. After doing so, we obtain the values:
Vr2 = 8.510 m/sec
Nr2 = 36.2 revolutions
L2 = 198.919 in.
This is the distance roller two travels using only smooth rotation at a higher speed — without any skidding. In this solution, we observe that roller two’s speed is higher than roller one’s speed, demonstrating that both rollers can turn freely without any drag or skidding.
So what do the results mean?
Refer back to the figure titled Skidding zone detail. In the first model using the single-roller design, all points on the roller were forced to travel at the same constant speed, causing the roller to skid a maximum distance of 6.6 in. to cover the distance.
In contrast, using two rollers allows both rollers to independently rotate with different velocities as well as travel different distances to cover the same angle traversed by the turntable. The crowned design provides for a point contact, and this outcome relieves friction-related stresses on the cam followers, reduces wear and tear, and extends the components’ service life.
Intech Power-Core | intechpower.com
Filed Under: Motion Control Tips
