【正文】 in advantages for a given type of belt. Tensiondeflexion1. Place a straight edge across the top ofboth pulleys. See Figure 35.2. Measure the span length.3. Using a spring scale at right angles tothe center of the span length, apply aforce to the scale great enough to deflectone of the belts the equivalent of times millimeters of span length (times inches of span length).4. The force should approximate the forcesshown in Table 34.Figure 35 Tension deflection methodItemDesignation1Straight edge2 1/2 Span length3Span length4Deflection mm (1/64) for each 25 mm (1) of spanlength5Pull6Motor pulley7Belt8Spring scale9Force (per table)10Crusher pulleyExampleVbelt section = Standard 8V beltSpan length (center distance) = 1575mm (62)Deflection force (from table) = 1522 Kg(3450 Lbs.)Deflection = 1575mm (62) x = 25mm(1)Therefore, on a NEW installation with astandard 8V section belt and a span length of1575mm (62), the belt should deflect 25mm(1) with a spring force of 1522 Kg. (3450Lbs.) for a properly tensioned drive.5. At the end of 24 hours of operation, thedrive should be retensioned toapproximately the maximum force.6. After 2448 hours, it is well to check thedrive to see if the force on the belts isbetween the minimum and maximumforce shown in table.Retension if necessary. Either excessively lowor high tension will affect the life andoperation of a Vbelt.IMPORTANT!When properly tensioned a narrow vbelt maynot feel as tight as one might expect from thetension they carry. Therefore, it is advisable touse a spring scale to check belt tension on thesebelts.SectionStandard beltBanded belts (*)Minimumtension kg(Lbs)Maximumtension kg(Lbs)Minimumtension kg(Lbs)Maximumtension kg(Lbs)8V15,4 (34) 22,7 (50)18,1 (40)27,2 (60)SPC10,4 (23) 15,4 (34)11,8 (26)18,1 (40)(*) Multiply these values by number of belts in band.Table 34 Flexion forces Elongation methodFor tensioning the drive using the PERCENT(%) OF ELONGATION method, proceed asfollows. This particular method of tensioningVbelt drives was developed primarily fortensioning banded belts.1. Remove the slack from the belts.2. Next, wrap a 15 meter (50 foot) steeltape around the outside circumferenceof the belt and measure to the nearestmillimeter (1/16). Record this length.Figure 36.3. Multiply this recorded length by apercent of elongation factor which isshown in Table 35 and add this amountto the initial measurement.4. Elongate the belt to this new reading.Figure 36 Percent of elongation methodItemDesignation1Tape reading2 Steel tape3Pull4Motor pulley5Belt6Crusher pulleyExampleVbelt section = Standard 8V beltInitial tape reading = 3048mm (120)Elongation factor (from table) = (multiplier) which is % (percent)New length reading = 3048mm (120) initialreading x elongation factor= 3048mm (120) + 27mm (11/16) or3075mm (1211/16).Belt section* MultiplyPercent (%)8V0,0090,9(*) Standard or banded beltsTable 35 Elongation factorsTherefore, on an installation with an 8V beltand an outside circumference measurement of3048mm (120), the motor pulley should bemoved away from the crusher pulley until thesteel tape reads 3075mm (1211/16) for aproperly tensioned drive.5. Periodic belt tension checks must bemade and if retensioning beesnecessary, the tensioning process justdescribed should be repeated.It must be pointed out that theTENSIONDEFLECTION METHOD is thepreferred method of tensioning Vbelts.Usually the first sign that indicatesretensioning is necessary is belt slippage. Thisslippage will show up in loss of power andspeed at the crusher pulley and in a high rate ofbelt and pulley groove wear. These conditionsare usually acpanied by belt 171。squeal187。 andalso a heating up of belts and pulleys. Theseconditions are readily apparent and can bedetected by simply looking, listening andfeeling.Vbelt precautionSome of the do’s and don’ts in connection withproper Vbelt drive operation are as follows:1. Do. Check belt tension frequentlyduring the first few days of runinoperation. When the belts have had timeto bee seated in the pulley groove,retension the belts. New belts have acertain amount of initial stretch and willrequire additional tensioning.2. Don’t. Overtighten belts as too muchtension shortens both belt and bearinglife. Maintain uniform tension. Idle beltsshould appear tight。 in motion, theyhave a slight sag on the slack side.3. Do. Keep drives well ventilated as heatbuildup over 60176。C (140176。F) causes beltlife to bee shortened. The sides ofthe belt guard must be designed to allowfor adequate circulation of air. Eitherperforated plate or expanded metal isideal for this type of application.4. Don’t. Allow any oil or grease to ein contact with the belts as excessive oilcauses the rubber to swell and the beltsto fail prematurely.5. Do. Make Vbelt drive generalinspections on a periodic basis. Thefollowing points must be checked ateach inspection.a. Loss of crusher speed checktension.b. Unequal stretch check each belt.c. Excessive elongation check foroverload.d. Belt softening or swellingcheck foroil or grease.e. Belt hardening and cracking checkfor excessive heat. Drive safety guardThe drive guard protects the operator and thedrive from accidental rock falls.As the motor position is specific to eachcrusher installation, the guard is often designedand produced specifically for each crushingstation.DANGERFor the safety of the personnel the driveguard must be built in pliance withcurrent safety regulations for the site on