Rotary positive displacement blower equipment (Gardner Denver, Sutorbilt, DuroFlow, etc.) are fairly simple mechanical devices. They consist of two rotors (2 or 3 lobes each) connected by timing gears, rotating inside a cylinder or housing. Their spinning moves air from inlet to outlet, pumping against the system backpressure. Stationary blowers are normally driven by an electric motor through a flexible coupling, v-belt drive, or combination (jack-shaft) drive. Mobile units are commonly driven by PTO from the prime mover drive shaft.
However, while the air is classified as “oil-free”, the blowers do require lubrication for the gears and bearings.
As mentioned in another posting one of the most frequent causes of blower failures is from excessive overhung load. This will primarily affect the drive shaft bearing which takes the brunt of the side loading on the shaft. This can also be transmitted through the rotors to the gears resulting in premature wear and subsequent failure of the gears and gear end bearings. Excessive loading on the drive shaft can also cause the rotors to make contact with each other (“knocking”) and to make contact with the bearing housings (head plates) and the cylinder.
Another frequent failure is from lack of lubrication. Rotary positive displacement blowers require adequate and proper lubrication for all bearings and the timing gears. Generally the gears and gear end bearings are oil splash lubricated. The drive end bearings can be either grease lubricated or oil lubricated (dual splash lubricated). The blower operation manual will specify the amount, quality, and viscosity of the lubricants best used in the blower. Typically, the oil will be a long life synthetic lubricant, and specifics vary due to operating and ambient temperatures.
Inadequate lubrication failures take one of three forms: no oil or not enough oil; too much oil; too long between oil changes and greasing of bearings. Most blowers are shipped without oil, but with tags indicating oil must be added prior to start up. We see a number of brand-new blowers brought in for repair where no oil was added prior to running the blower. This results in catastrophic failure of the gears in short order.
The destructive pattern of the gears is a tell-tale sign of lack of lubrication. Loss of lubricant through leaks or other means can result in overheating and failure of the bearings and gears. Over-filling the oil sumps can also result in failure due to excessive foaming. The failure pattern of the gears is very similar to that of no oil. Infrequent maintenance results in breakdown of the oil and loss of lubricity with subsequent overheating and failure due to premature wear on bearings and gears.
The third most frequent failure in positive displacement blowers is due to over-pressure. An inadequately sized or failed relief valve is one cause of overpressure issues. Overpressure generally results in overheating and warping of the rotors resulting in rotor-to-rotor contact near the center of the lobes. The rotors will subsequently make contact with the cylinder and/or the head plates.
Of course there are other failures, and occasionally a warrantable failure due to manufacturing defect or defective part. These failures if they occur within the manufacturer’s warranty period will be handled by the manufacturer. Pye-Barker Supply Co. and Gardner Denver have an excellent record of handling warranty issues, fairly evaluating every failure with an eye toward customer satisfaction
Blowers are dumb beasts.They do not know nor care what is driving them. They just rotate and move air.
There are a number of methods of driving a rotary positive displacement blower.
The simplest and preferred method is a simple flexible coupling, driving the blower at the same speed as the motor. Woods Dura-Flex and Sure-Flex couplings are examples. They, of course, must be properly sized and rated for the motor horsepower, the load, and the speed. Certainly the use of a flexible coupling is limited to matching the motor speed. Since a blower operates on a curve, the optimal blower selection will most likely require an input speed that does not match the motor speed.
When the motor speed does not match the required speed of the blower, a v-belt drive is needed. A number of factors are used to properly size a v-belt drive, including the motor horsepower and speed, service factor (1.4 for rotary positive displacement blowers), blower required speed and horsepower, motor and blower shaft diameters, and maximum allowable “overhung load” on the blower shaft. Of equal importance is the location of the blower sheave on the shaft. In general, the sheave must be placed as close as possible to the fact of the blower, in some cases no further away than 3/8 inch.
The most frequent failure we see in rotary positive displacement blowers is the drive shaft bearing. This is the bearing on the input shaft on the main rotor. With a v-belt drive, this bearing is susceptible to the greatest stress of any of the bearings. In addition to carrying the weight of the rotor and the rotational forces imparted to it, it also has the side load created by the v-belts. If the belt drive is not properly sized and located, it can place unacceptable overhung load on this shaft and its bearing.
All Gardner Denver blower manuals (Sutorbilt®, DuroFlow®, HeliFlow®, & CycloBlower®), and most, if not all, blower manufacturers includes a section on the drive sizing, with charts and formula to calculate the overhung load and insure it is within allowable limits. These equations are pretty simple math, but the load data from the v-belt drive must be provided in order to calculate these factors.
V-belt drives for rotary positive displacement blowers should be sized based on a 1.4 service factor. This ensures the integrity of the drive itself, and makes sure the drive is adequate. A lower service factor can result in the belts “rolling” and slinging off the sheaves. A service factor too high can increase the overhung load on the blower shaft and bearing.
A third drive method utilizes both a flexible coupling and v-belt drive. This is called a jack-shaft drive, with the motor driving through a v-belt drive to a jack shaft (properly sized shaft and pillow-block bearings). This shaft then drives the blower via the flexible coupling. In this method, the excessive overhung load on the blower shaft is avoided by direct driving the blower, but flexibility in speed is maintained by use of the appropriately sized v-belt drive.
Pye-Barker Supply Co. has the expertise and resources necessary to properly size the various drives and calculate the resultant overhung load on the blower shaft. We also have contacts in the industry to assist in designing the proper jack-shaft drive as needed.
One of the major problems facing industry today is the limited number of people with sufficient skill and experience to diagnose and rectify the basic problems plaguing positive displacement pumps. Another difficulty is that the same lack of skill and experience is creating many of these problems in the first place.
A detailed evaluation of a pump problem requires a depth of knowledge which usually surpasses that to which most people are ever exposed. Most pump engineers, operators and maintenance people develop their knowledge base from the same "school of hard knocks."
While this on-the-job type of training has much to commend it, unfortunately it also exposes people to the opportunity of learning the mistakes and misconceptions of others. At best, it only teaches what is necessary to execute a particular job function in exactly the same manner as it was previously performed-good or bad!
The ramifications are generally imposed on the maintenance department, where the training is usually limited to the physical change-out of parts when a breakdown occurs. As the underlying cause of pump failure often extends beyond the failed item, these maintenance methods will effectively reinstall the same old problem.
This is particularly concerning when we realize that over 80 percent of all pump failures tend to manifest themselves at the mechanical seal or the bearings. Typical mechanical seal failures depend highly on the seal type and material pairing. O-ring-type shaft seals with dynamic O-ring and one seal ring in carbon-graphite typically have problems with wear on seal faces and seal hang-up, which prevents axial movement of the dynamic O-ring and seal ring.
Mechanical shaft seals with hard/hard seal face material pairings usually experience problems associated with dry running and bearing failure.
When a bearing fails, it is important to determine the exact cause so appropriate adjustments can be made. Examination of the failure mode often reveals the true cause of failure. This procedure is complicated by the fact that one failure mode may initiate another. For example, corrosion in a ball race leaves rust, an abrasive, which can cause wear, resulting in loss of preload or an increase in radial clearance.
The wear debris can, in a grease-lubricated bearing, impede lubrication, resulting in lubrication failure and subsequent overheating. When this happens, it will transfer to the mechanical seal causing the two seal faces to walk on one another which will cause seal failure.
Both bearings and mechanical seals act in a manner similar to a fuse in an electrical system. When a fuse in an electrical system fails, it does not mean there is anything wrong with the fuse. In fact, we understand that the problem is almost always somewhere else in the system.
Despite this, when a seal or bearing fails, we rarely look for the real problem. Instead, we simply replace the offending part. While that will occasionally solve the problem, simply replacing a seal or bearing rarely provides long-lasting relief from the true issue.
The extent to which this happens varies from industry to industry, as some are more aware of the root causes of pump failure than others.
We frequently receive calls for an air dryer. When pressed for specifics, the customer often has no idea how dry he needs the air to be. He may only need to remove liquid water and contaminants from his line. Or, he could need “instrument quality” air.
The amount of moisture in compressed air is expressed as pressure dew point (PDP), and is given in degrees Fahrenheit. The dew point is the temperature at which the moisture in the compressed air will condense and form a liquid. Since compressed air at 100 PSIG is saturated (100% relative humidity), some form of moisture removal is generally necessary in a compressed air system which means reducing the dew point, condensing the moisture, and removing the liquid water. In most systems, this is done using one or more of the following methods:
Pye-Barker Supply Co. represents several manufacturers of premium quality air drying and cooling equipment. These can be specified to meet most any cooling/drying requirement.
