Towlines, or tugger cart systems, are a series of carts that a motorized vehicle pulls around a facility. They are the lifeblood of many different companies. Without towlines, the whole operation will break down. These towlines are composed of carts, and on these carts, casters connect the whole operation to the ground. If these casters fail, the whole operation fails. Because of how important casters are, we need to make sure that the casters will handle the unique requirements of towlines. You need to consider four main issues when choosing casters for tugger systems.
1) LOAD RATING ("THE RULE OF 3") in towlines
Our clients need to know precisely how much weight each caster can hold. We rate each of our casters and conveniently post the ratings per caster in our catalog and online, which are all rated a 3mph, or “walking speed.” However, speed plays a huge role in towlines because the load capacity of your casters decreases, the faster the cart moves. The load rating decreases due to the heat build-up with the increased speed and the revolutions per minute.
Your cart may also use only 3 wheels on uneven surfaces at various times during transport. For these reasons, we use the “rule of 3” to calculate your load capacity. The rule of 3 says that when you calculate initial weight capacity, you only calculate weight capacity for 3 of your wheels, not 4 because we need to build in a safety factor due to uneven floors, inclines, and declines. This is a fast way of calculating required load capacity per caster. Here is the basic formula, but see in section 2 for the effect of speed on load rating.
C = (L+W) ÷ 3
C = Required Load Capacity of Each Caster
L = Maximum Cart Load
W = Cart Weight
For example, consider a 300-pound cart with an 1,800-pound load on it. This would be a 2,100 lbs. divided by 3. Each caster would have to support/be rated for 700 pounds or more for this load.
700 = (1,800 + 300) ÷ 3
2) SPEED IN TOWLINES
SPEED'S EFFECT ON LOAD
Speed plays a huge role in calculating load rating. Now that we have determined the initial load rating of a caster with the rule of 3, we need to consider the speed factor in order to properly determine the load rating to ensure the wheel tread does not fail.
For example, take our cart above that weighs a total of 2,100 lbs. We have determined the dolly travels at 5 mph for an ANSI duty cycle of 2 minutes on and 4 minutes off. Due to the increase in speed to 5 mph, the caster polyurethane tread loses 20% of the catalog rating, so we need to add 20% to the load rating of our caster. That means that the 700-pound caster rating ACTUALLY needs to be 840 lbs. minimum to be successful.
Dividing the total load by 4 (rule of 3) and not adding 20% (speed factor) could create a disastrous result!
SPEED AND DUTY CYCLE
Towline casters have to be able to handle the extra speed of towlines. You may have a caster that is excellent for ergonomics, but there is no guarantee that this caster will work for towlines. In fact, many times an extremely good ergonomic caster will not work for towlines at all. In other cases, you get the versatility of having a caster that performs well in both applications. Make sure you take into consideration the speed of the towline when selecting casters.
Duty cycle is an often-overlooked consideration with towline casters. Whenever you have casters moving within a towline, heat is generated on the wheels and rig. This heat is released at its greatest form right after the caster stops its motion. Your casters must remain at rest for a bit of time until they have eventually cooled to the point that you can begin the cycle again. This is the duty cycle. Stronger and smoother bearings as well as aluminum cores will aid in heat dissipation and allow you to spend less time waiting for your wheels to cool.
As you can see, speed and duty cycle are huge factors in towlines. We help our clients calculate their load capacities for their towlines. In this way, we make sure we have accounted for this speed to eliminate tread failure while also ensure that you have the proper wheel to handle the duty cycle you need to adhere to.
3) TREAD AND BEARING TYPE FOR TOWLINES
Because of the combination of weight and speed, you have to choose your tread wisely. We recommend towing treads on the shore-A durometer scale because we need the compression of the tread for traction. Our standard recommended material for towlines is polyurethane because polyurethane grips the ground, absorbs shock, resists tearing, and repels debris.
Towlines that use hard wheels on the shore-D scale create a major sliding hazard as well as tremendous noise and floor damage. Examples of materials that are found on this scale are nylon, steel, and phenolic. These hard wheels are extremely dangerous in towlines, especially when tuggers are making 90-degree turns. The inertia of the rear cart(s) will cause these dollies to slide hurting employees and damaging equipment.
Make sure you choose the right tread!
The bearings found in your caster wheel are hugely important as well. Many clients have casters that use caged roller bearings. This is a mistake since these bearings will not hold up under the constant speed and high rotation of caster wheels in towlines. We use precision ball bearings, which are many times larger and are therefore much more durable under the rigors of a towline.
Make sure that you have the correct tread and bearing type for your casters so that you avoid potentially huge problems in the future.
4) SWIVEL fork and raceway TYPE
When our clients design carts for their towlines, we help them select the right swivel fork and raceway.
The swivel fork is an easy one. You need your fork to be an extended lead so that you don’t get any flutter in your caster wheel as you move at higher speeds. Flutter–you’ve probably experienced it in the supermarket–is when your wheel flutters back and forth as you move the cart. When you extend your caster’s lead, this gets rid of the flutter because you are giving the caster room to trail properly.
Your raceway is extremely important in towline casters. First, you need a rig that is durable enough to handle the constant motion and turning of the towline. The design of the rig raceway will determine whether or not your rig can handle the stress of being in a towline. For example, a kingpin rig with its kingpin and ball bearings cannot handle the stress and rigors of towing speeds and forces. The kingpin will stretch and/or snap, causing the swivel raceway to fail. When your caster raceway fails while you transport products in a towline, you risk injury to employees, damage to goods, and downtime in general.
To remove this failure point of the standard kingpin, we recommend using a kingpinless raceway (we remove the pin so we remove the failure) or a forged kingpin raceway. Simply put, there are towline rigs and there are not towline rigs. Kingpinless and forged kingpin caster rigs are your options here. We highly recommend that you not use a standard kingpin caster for towlines. You may save a few bucks upfront, but you will spend MUCH MORE in repair cost, downtime, and maintenance labor. You’ll also happen to prevent injuries if you’re also pushing or pulling your dolly by hand, but we’re focusing on towlines here.
3 MOST IMPORTANT CASTERS FOR TOWLINES
Now, that you’ve got an idea about the characteristics of a great towline caster, you can see the differences that exist in our 3 most important casters for towlines: the CC Apex, CC Apex HD and CC Stark. These casters differ from each other in important ways.
We have to say that all three of these casters will perform extremely well in towlines and as an ergonomic caster as well. That means that you can push and pull these casters at any stage in the towline with world-class ergonomic performance, while also using them in towlines. In this light, you have total versatility.
First, the CC Apex is our most versatile caster wheel and has a donut tread. Because of its donut tread, it is superb with debris and impediments while also being extremely quiet. Its aluminum core dissipates heat slightly better than iron, which means that you can reduce the time you need to allot for your duty cycle.
You can pair this caster with an extended lead kingpinless (CC Alpha) or a maintenance-free forged kingpin (CC Peak), which is extremely smooth for both towlines and ergo applications. Overall, you get an extremely versatile, smooth, durable, and quiet caster with the CC Apex and its matching rig.
CC APEX HD
Second, you have the CC Apex HD, which essentially is the CC Apex in larger sizes and with higher weight capacities. The CC Apex HD allows you to move more weight in a single trip and is a bit quieter than the CC Apex because of its thicker and bulkier donut-tread wheel. Although it has an iron core for greater weight, it still dissipates heat very well. If you need larger caster wheels and/or have higher loads, this caster wheel with either a forged kingpin (CC Peak), kingpinless (CC Alpha), or a CC Armor (larger version of the CC Alpha) is your best choice.
Finally, the CC Stark is superb for speed with heavy weight. The CC Stark has a slightly crowned tread so you get less compression and are thus able to move more weight. It also comes with its own unique rig, which is a forged kingpin. Although it’s also superb for ergonomics, you lose a little versatility that the donut tread of the CC Apex and CC Apex HD offer you.
However, this caster is bombproof, is more or less maintenance-free, and carries with it the best warranty in the industry at 4 years. If you want fantastic ROI and want the best caster for towlines, then this is your caster. In this industry and the abuse that casters face in any towline, a 4-year guarantee will give you peace of mind, knowing that you don’t have to change casters for at least 4 years.
There are a few slight differences that we discuss with our clients, so that they get exactly what they need, but these are by far and away the most important casters and characteristics for towlines. We put together individual plans for our clients and would love to do the same for you. Feel free to click below if you would like a partner to work with you and get you the exact product for your towline.