Exmark Original Belts
Why Should I Buy Exmark Original Belts?
Engineers have to start here. The service factor of a drive reflects the overall degree of difficulty for any belt put into the application. Is the distance between pulleys very short? That will increase the service factor. Are the pulleys very small? That also puts added stress and heat on any belt applied. Does the belt have to absorb any shock loading? Are the pulleys misaligned?
Unlike the classic "horseshoes and hand grenades" principle – simply getting close to the target is not good enough when it comes to the right belt dimension. Exmark designs for only 2% take-up tolerance, leaving very little tolerance for error. This becomes even more difficult to achieve as the belt experiences a certain amount of stretch during break-in.
Don’t let anyone fool you that some belts don’t stretch during break-in. Even wire cable can be stretched. In the case of v-belts, the prescribed amount of elongation depends mostly on the type, diameter and twist of cord in use. As a general rule of thumb, aramid cord (commonly referred to as "Kevlar") will stretch about 1% in application. Polyester cord can stretch as much as 2-3%, requiring the designed dimension to be right on the money.
The belt cushion is the area of rubber under the tensile cord, toward the bottom of the belt. In our industry, two primary materials are used, often in a blend with natural rubber and reinforcement fiber. SBR (Styrene Butadiene Rubber) is the aftermarket rubber of choice due to its low cost. SBR works for a while, but tends also to crack and fail prematurely due to its low tolerance for elevated temperatures. Exmark uses SBR only in low speed applications. Our material of choice for v-belts is neoprene. It may cost more, but neoprene’s tolerance for higher temperatures makes it a far better choice. Neoprene does a great job of maintaining its flexibility over a higher temperature range.
Also used in the cushion material of a belt is reinforcement fiber. Belt manufacturers process these barely visible fibers to align in one direction within the cushion. As a result, the fibers help to prevent the belt from flexing laterally. Without that resistance, belts can seat in to the pulley too deep, causing belt slippage and increasing the heat generated in operation.
Tensile Cord Selection:
The two most commonly used cord materials in our industry are polyester and aramid (Kevlar). Aftermarket companies usually tout aramid as being the best choice for cord material due to its great tensile strength. While that may be true in general, the specific service factor and pulley diameters for your drive may require a more flexible polyester cord. More importantly, aramid’s toughness also makes it more abrasive on itself, thereby increasing friction and heat inside the belt. Polyester’s smoothness minimizes that friction. In a drive with many bends, a smoother, more flexible polyester cord may be the engineer’s best option.
Tensile Cord Diameter:
Along the same lines is cord selection. Choosing the correct diameter can make or break your belt life. Generally speaking, small diameter cord gives better flexibility for bending around tight pulleys, but may not provide optimal strength. Conversely, larger diameter cord gives greater strength, but may not be the right choice for a tight bend radius.
Tensile Cord Placement:
In applications where misalignment, or vibration and oscillation create challenges, a lower cord line (the distance from the bottom, or narrowest portion, of the belt vee to the top, or widest portion of the belt) helps prevent the belt from rolling over in the pulley. At the same time, drives requiring the belt to bend backwards may require a slightly higher cord line. Generic belts usually have a standard cord placement in some variance to the optimal location, diminishing effective life.
Tensile Cord Twist:
How tightly, or compact, that cord winds makes a big difference in its ability to handle bending and shock loading. Much like a spring, a more compact cord twist enables the belt to absorb greater shock loads. The down side to increased twist of course, can be an increased tendency to elongate in operation, while lower twist is stronger with decreased flexibility.
Most belts in mower applications use an envelope, or "wrapped" construction. In these cases, the belt carcass (cords and cushion) is wrapped with a fabric designed to act as a clutching component in the belt drive. How much friction is required for the belt to grab the pulley in a shock load (for example, when the operator pulls the PTO switch to engage a deck drive), depends on how much carcass rubber impregnates, or "bleeds" through the windows of the envelope fabric. The greater the rubber bleed through, the more the belt will grab. Engineers closely control this component to ensure the right balance of friction while avoiding excessive heat build-up.
The next time your customer balks at the retail price of a deck or pump belt, and
threatens to buy a much cheaper industrial, or aftermarket product, consider this.
Aftermarket companies have the luxury of only having to design belts to a generic
dimension, with generic materials. The trouble with that is, nobody operates a generic
machine. Every belt drive has its own dynamics and unique demands. The belt you
use on that drive can only answer the call if the same engineering team that designs
the machine, designs the belt. Otherwise, your customer may only get the performance
and reliability they paid for.
Arm yourself with the facts. Here are just some of the design elements taken into consideration before a v-belt gets Exmark’s approval:
- Service Factor
- Belt stretch
- Cushion material
- Fiber loading
- Tensile cord selection
- Tensile cord diameter
- Tensile cord placement
- Tensile cord twist
- Envelope fabric